I think Mertil was mentioned again the book where they get the Auxileries, but only in passing.
But I don't think book 40 was meant to be one of the "important" books of the series. It was mostly filler. The only thing that passed onto later books was the term "vecol", and Ax's attitude reguarding handicaps.
Which is a bit of a shame, really. I loved Gafinilan and Mertil.
I imagine Mertil would have stayed hidden after the war was over. After all, he would want to be remembered as a hero who died in battle, not as a vecol. Marco might have made good on his promise to hang out and play video games, though.