No dice. CPls are DLLs that export a function - CPlApplet(). winegcc coughs on dllexport. - not implemented. I've been hacking at it for days on end now. Result: CPl can be compiled under windows and run under wine. CPls compiled with winegcc don't export that function => not treated as control panel applets.
I'm sure it must be possible to export routines in Wine .dll.so libraries via .def files or similar - whether GetProcAddress supports Wine "DLLs" is the issue. Even if for some reason it's not possible, I don't see that it would be impossible to detect that the .cpl file is actually a Wine DLL, and to use the appropriate dlopen-type routines to read it.