The localization would be found out by histochemical GUS assay. As the assay depends just on the promoter sequence of the gene, so would it be right to speculate that both the proteins might localize to similar tissues, although in different plants?
Depends on if there is a conserved subcellular localization signal. GUS fused to a genes promoter indicates the tissue in which it is expressed. Subcellular localization in dependent upon the protein sequence. There are programs which can predict subcellular localization based on the protein sequence.
since your question is not about the particular subcellular localization, but about the transfer of knowledge from Ath to Brassica: the SPECULATION or hypothesis is definitely correct, which does not mean that this will exempt you from confirming this in case there are doubts in the interpretation of results. Although the previous answer from Craig could be helpful in a sense that you could particularly check whether the predicted localization signals are highly conserved as well.