Well, yes, there is a better way!... at least for XS, I don't know about Inline::C, though probably not

Not that I've applied the "better way" (in either XS or Inline::C), but I would expect that Inline::C *does* lend itself to this improvement.

There's certainly no problem in Inline::C as regards defining macros - nor would I expect any problem with using pTHX, aTHX, pTHX_ and aTHX_. (But I don't know for sure :-)

Cheers,
Rob