Emulating Windows __thiscall in C

Does anyone know to call a __thiscall function from C code? It's not a problem of garbled symbols (I have its address), it's a problem of passing this pointer in ecx. I thought declaring function as __stdcall and storing stuff in ecx with assembly magic would work, like this:
    __asm { mov ecx,self }
    res=sub(a2,a3,a4,a5,a6,a7);

But code generated by visual studio overwrites ecx:
dinput8.dll:66551153 mov     ecx, [esp+10h] # my code
dinput8.dll:66551157 mov     ecx, [esp+120h] # automatically generated by msvcs, thanks a lot
dinput8.dll:6655115E mov     edx, [esp+11Ch]
dinput8.dll:66551165 push    esi
dinput8.dll:66551166 push    edi
dinput8.dll:66551167 push    ebx
dinput8.dll:66551168 push    ebp
dinput8.dll:66551169 push    ecx
dinput8.dll:6655116A push    edx
dinput8.dll:6655116B call    off_66553344

Any ideas? I'd even be fine with writing some fake C++ class and calling its method, but unfortunately you can't specify an address of method of class when calling it...

>>5
So you just want to make a stdcall or  cdecl(but not fastcall) convertor to thiscall?

Here's my quick attempt at it:

push ...
push ...
call thiscall_func
->
push ...
push ...
call your_stub_for_thiscall_func


your_stub_for_thiscall_func:
add esp,4
pop ecx
push dword ptr ss:[esp-8]
jmp thiscall_func

My stdcall/cdecl->thiscall convertor approach is clean in that it doesn't really use any extra registers or extra stack space.