@nsivov: tried it, but the lpszName field of CREATESTRUCT is a copy of a the passed string (as lpszClass is). (it seems to be the direct pointer in hooks though, but I don't think it's a good idea to use hooks for msvfw32 implementation)
The only other solution I see is to set a global variable when entering MCIWndCreate and test for it in WM_CREATE handling Yes, that's ugly. If you have better ideas...
I extended the tests for windows from MCIWndClass (see MR!776)