Yaxley did turn up in HBP - mentioned in the Spinner's End chapter, iirc. Which is fair enough - twelve DEs, most of whom we'd heard of before, got taken off to Azkaban at the end of OotP and Voldemort was letting them stew a bit for punishment, so JKR did need some new names. And she did go back to Dolohov and Travers, at least.

Dammit, someone's agreeing with me! I was just ready to feel all daft and pathetic for minding that she wasn't keeping proper account of her one-note villains!