Happily, passing doubles or long longs is very rare in Atari
code. And until the 16-bit extension bug is fixed, I would
suggest VBCC users just to use 32-bit.
So if I understand it correctly, this is a problem only for VBCC's "-mshort" + double/long long? If so, it's really rare that somebody would use 16bit compiler for such numbers. But as general rule, doubles are used quite a much, take a look at any of my ports (quake specially).
Btw do I understand it correctly that you plan to have 16 bit gemlib + vbcc's libc as the base stack for GEM apps (for developers who, for some reason, prefer to use -mshort) ? If so, is there any point except "we can" ?
Anyway, nice initiative, I'm glad you continue to support Frank with Atari inputs, I enjoyed cooperation with him very much.