I suspect that once the classes exist, games will expect them to actually work, and adding the stubs will break some games that currently work.
To avoid this problem, it should be possible to add option enabling these classes support in compilation configuration or Wine's config panel. Has this idea any sense?
If we really wanted to disable the classes by default, the way to do it would be to not register gameux.dll by default. You could then run regsvr32 gameux.dll to set them up for a Wine prefix, until it's considered good enough to be enabled by default.
But I don't know if that sort of thing would be accepted in Wine, or if it would be better to enable it from the start.