The thread title is very confusing really, given that Tex Mex, cajun and zydeco represents to many the finest flowering of the diatonic accordion in the world.
   A lot of players of diatonic accordions in England happily play along with what might be called "bluegrass" music(in the wider sense I suppose).Tony Weatherall springs to mind(both on the cajun accordion and the Tex Mex style Hohner D/G)who is a great player on general American music, as well the narrower field of cajun.