I am a Japanese native speaker. From my point of view, the following sentence sounds strange (ungrammatical).

a. Taro wa [dare-ga ki-mas-i-ta to] it-ta ka?

If you want to put -mas- form in some place of the sentence in question, it will be like the following.

b. Taro wa [dare-ga ki-ta to] ii-mas-i-ta ka?

Hiromi Yamamura

 Dear Francesc González i Planas:

I am Hiromi Yamamura, who  answered your question about one hour ago.

I send this mail to add something to my  previous answer.

In my answer, I wrote the following.

"If you want to put -mas- form in some place of the sentence in question, it will be like the following.

(b) Taro wa [dare-ga ki-ta to] ii-mas-i-ta ka?"

What I want to add is that -mas-form in (b) does not express Taro's polite attitude to the hearer but the speaker's attitude to the hearer. If you want to translate an English sentence "Who did Taro say came ?" into a non-polite Japanese, it will be like

(c)  Taro wa [dare-ga ki-ta to] it-ta ka?

Apart from the sentences cited above, there is a sentence like

(d)  Taro wa [dare-ga ki-mas-i-ta ka to] it-ta ka?

The sentence (d) means "Did Taro say who came ?"  The -mas- form in (d) expresses Taro's polite attitude to his hearer. If you want to add your polite attitude to your hearer, it will be like

(e) Taro wa [dare-ga ki-mas-i-ta ka to] ii-mas-i-ta ka?

Now, when you convert an assertive sentence like "Taro wa [Hanako-ga ki-mas-i-ta to]it-ta" into an interrogative sentence, what will happen ?

(a) Taro wa [Hanako-ga ki-mas-i-ta to] it-ta.

(f) Taro wa [Hanako-ga ki-mas-i-ta to] it-ta ka?

The -mas- form in (f), which means like "Did Taro say Hanako came?", expresses Taro's attitude to his hearer. If you want to convert [Hanako-ga ki-mas-i-ta] to [Hanako-ga ki-mas-i-ta ka], the sentence will be like

(g) Taro wa [Hanako-ga ki-mas-i-ta ka to] it-ta ka?

The sentence (g) means literally "Did Taro say if Hanako came?", but a natural translation will be like "Did Taro ask if Hanako came?"

If you want to add your polite attitude to your hearer, the sentence (g) will be like,

(h) Taro wa [Hanako-ga ki-mas-i-ta ka to] ii-mas-i-ta ka?

That is all.  I hope my answers were useful for you.

Hiromi Yamamura 

I am not a Japanese speaker. But I wonder if (h) 'Taro wa [Hanako-ga ki-mas-i-ta ka to] ii-mas-i-ta ka?' in Prof. Yamamura's response is not a direct quotation like: Did Taro ask, "Did Hanako come?"? In that case, the sentence should actually be: Taro wa "Hanako-ga ki-mas-i-ta-ka?" to ii-mas-i-ta ka?

As far as I understand, -mas- form in Japanese is an addressee/hearer-honorific, so it cannot be used in an embedded clause UNLESS it is a direct quotation. That's why Prof' Yamaura's (d) 'Taro wa [dare-ga ki-mas-i-ta ka to] it-ta ka?' and (e) 'Taro wa [dare-ga ki-mas-i-ta ka to] ii-mas-i-ta ka?'could only mean Did Taro say/ask "Who came?"

Is my understanding correct?

I also wonder about Prof. Yamamura's (b) 'Taro wa [dare-ga ki-ta to] ii-mas-i-ta ka? ' 

Isn't this sentence actually ambiguous, at least in a written form?:

(i) Who did Taro say came? and (ii) Did Taroo say/ask, "Who came?"?

Dear Hyo Lee:

Thank you very much for your response. You are right when you point out that the correct direct quotation sentence should be Taro wa "Hanako-ga ki-mas-i-ta-ka" to ii-mas-i-ta ka ?

If you translate an English sentence like "Taro asked if Hanako had come" into a Japanese sentence, it will be like "Taro wa [Hanako-ga ki-ta ka douka] wo tazune-ta". So, your response is correct.

"As far as I understand, -mas- form in Japanese is an addressee/hearer-honorific, so it cannot be used in an embedded clause UNLESS it is a direct quotation. That's why Prof' Yamaura's (d) 'Taro wa [dare-ga ki-mas-i-ta ka to] it-ta ka?' and (e) 'Taro wa [dare-ga ki-mas-i-ta ka to] ii-mas-i-ta ka?'could only mean Did Taro say/ask "Who came?" Is my understanding correct? "

Yes, your understanding is correct. 

"I also wonder about Prof. Yamamura's (b) 'Taro wa [dare-ga ki-ta to] ii-mas-i-ta ka? ' Isn't this sentence actually ambiguous, at least in a written form?:(i) Who did Taro say came? and (ii) Did Taroo say/ask, "Who came?"?"

I don't have much confidence in myself, but I have an impression that there is a little difference between the Japanese translation of (i) Who did Taro say came ? and that of (ii) Did Taro say, "Who came?"?  That is, 

The translation of (i) Who did Taro say came ? will be

(i') Taro-wa [dare-ga ki-ta to] it-ta ka/ii-mas-i-ta ka ?

(i")*Taro-wa [dare-ga ki-mas-i-ta to] it-ta ka/ii-mas-i-ta ka ?

(i''')*Taro-wa [dare-ga ki-ta ka to] it-ta ka/ii-mas-i-ta ka ?

The examples cited above show that the Japanese translation of (i) Who did Taro say came? is only (i').

And the translation of (ii) Did Taro say, "Who came?"? will be

(ii') ?Taro-wa [dare-ga ki-ta to] it-ta ka/ii-mas-i-ta ka ?

(ii")*Taro-wa [dare-ga ki-mas-i-ta to] it-ta ka/ii-mas-i-ta ka ?

(ii''') Taro-wa [dare-ga ki-ta ka to] it-ta ka/ii-mas-i-ta ka ?

The examples cited above show that the most appropriate translation is (ii''') and that the Japanese translation "(ii') Taro-wa [dare-ga ki-ta to] it-ta ka/ii-mas-i-ta ka ?" may be not ungrammatical (in that maybe there are many Japanese people that say that it is OK in a colloquial Japanese), but is not the best one.  

I have written "I don't have much confidence in myself" because Japanese linguistics is not my speciality. But, the difference between "(i') Taro-wa [dare-ga ki-ta to] it-ta ka/ii-mas-i-ta ka ?" and "(ii''') Taro-wa [dare-ga ki-ta ka to] it-ta ka/ii-mas-i-ta ka ?" is evident. If a Japanese hears  "(i') Taro-wa [dare-ga ki-ta to] it-ta ka/ii-mas-i-ta ka ?" , he/she will surely translate that to "Who did Taro say came?" , whereas, if the Japanese hears "(ii''') Taro-wa [dare-ga ki-ta ka to] it-ta ka/ii-mas-i-ta ka ?",  he/she will tranlate that sentence unhesitatingly to "Did Taro, "who came"? ?

Hiromi Yamamura

Dear Hiromi Yamamura and Hyo Lee:

Thank you for your answers. They are very interesting and useful for me.

Best regards,

Francesc González