it is a big topic itself and I'd like to discuss it separately.  Right now it's hard for me to see the rationale to require those info *only* for srfi-170 (or, foreign errors if we include srfi-198).

So you're thinking of expanding the scope beyond SRFI 198?  That indeed makes a lot of sense, but it's hard to see how it would avoid an indefinite delay in finalizing SRFI 198.  As in, while perhaps not quite a "boil the oceans" goal like a universal FFI, how do you think you could keep it from becoming a very big and invasive to existing Scheme implementations effort?