I do this

return(EXIT_SUCCESS)

And I don't care if you don't like it. It doesn't violate the standard. All you return EXIT_SUCCESS jerks can suck it.

1. is there a way to do that in standards-conforming (no GNU extensions) way?

GCC will exist since linux and most of open-source depending on it. Why you hate GNU/Freedom?

2. there's a lot of boilerplate here. couldn't we generate varapply/opapply macros automatically?

Unfortunately, no.
1.C preprocessor forbids recursion and attempts to circumvent it result in slow processing and inability to nest anything safely see http://stackoverflow.com/questions/12447557/can-we-have-recursive-macros
2. Inability to refer to concrete #place of argument in arg manipulation code. Recursion makes all #numplaces equal.