I know that __stdcall functions can't have ellipses, but I want to be sure there are no platforms that support the stdarg.h functions for calling conventions other than __cdecl or __stdcall.
The calling convention has to be one where the caller clears the arguments from the stack (because the callee doesn't know what will be passed).
That doesn't necessarily correspond to what Microsoft calls "__cdecl" though. Just for example, on a SPARC, it'll normally pass the arguments in registers, because that's how the SPARC is designed to work -- its registers basically act as a call stack that gets spilled to main memory if the calls get deep enough that they won't fit into register anymore.
Though I'm less certain about it, I'd expect roughly the same on IA64 (Itanium) -- it also has a huge register set (a couple hundred if memory serves). If I'm not mistaken, it's a bit more permissive about how you use the registers, but I'd expect it to be used similarly at least a lot of the time.
Why does this matter to you? The point of using stdarg.h and its macros is to hide differences in calling convention from your code, so it can work with variable arguments portably.
Edit, based on comments: Okay, now I understand what you're doing (at least enough to improve the answer). Given that you already (apparently) have code to handle the variations in the default ABI, things are simpler. That only leaves the question of whether variadic functions always use the "default ABI", whatever that happens to be for the platform at hand. With "stdcall" and "default" as the only options, I think the answer to that is yes. Just for example, on Windows, wsprintf and wprintf break the rule of thumb, and uses cdecl calling convention instead of stdcall.