I think this is highly unlikely. The main reason is that most trilobite species moulted several times after reaching what is regarded as maturity: their final morphology, with adult number of thoracic ribs. There are also quite a few cases of suspected nesting behaviour, where pairs of the same species (at similar growth stage) are encountered within a shelter such as a nautiloid shell; I've seen this is Wales, and it is common at some levels in Bohemia.

The individuals involved in this are normally not the largest examples known, or even close to it. Therefore, if these were reproductive pairs, then they could survive to moult several more times. Unless reproduction was unlikely at an early adult stage, semelparous trilobites should never have got to their full size.  

This might not apply to all trilobites, of course, and my wife's just reminded me about agnostids... which seem like a good candidate for semelparity, if there is one!

I agreed with above statement by Dr. Joseph Botting. Recent discovery of trilobite eggs is  better link to explain already recorded brood pouches in cephalon of trilobites. Of course, i am not the best person to tell your answer, but still personally feel that for better explanation of being semelperous behavior of trilobites needs thorough scrutiny of behavior and reproductive evidences of trilobite in fossil record, if available.

Definitely, you may proceed your phylogenetic experiment with best preserved trilobite species along with best characters all together. I am looking forward to have a look new insights from you. 

best 

Santosh    

I am in total agreement with, and appreciate, your feedback that semelparity is "highly unlikely." Though there is always something to learn by following unbeaten paths....I will plod along the phylogenetic path to see where it leads.