I expect that not to be the case - the executable format has very little to do with the calling convention - it may state that standard DLL's should use stdcall calling convention, but if you really want to you can create a DLL that uses fastcall, cdecl or thiscall calling convention - or any mixture of all four. The PECOFF manual describes how to relocate an executable file, how to find symbols in it, etc, etc, but not how to use assembler code - that is not part of the PECOFF format itself.
The fact that the executable contains functions is for most purposes irrelevant (and in fact, the executable may only contain one function that never calls any other function).
I have a feeling that Microsoft doesn't have a ABI specification. A quick search doesn't find anything at all.