I think it would be interesting to see how much of windows will run with wine's dlls in their current state. Obviously it would be bad to test this with something like user32.dll, etc, but something like riched32 and maybe some of the common controls dlls should be ok to test with?
There is a windows build available for wine(or used to be) where you can get PE builds of Wine's DLLs. I think Wine's mshtml.dll allows you to run Internet Explorer 6 using the gecko engine on Windows, but I never tried this.