Well the user never sees the C:\\Program Files as default he/she sees it as C:\\ABC
Then when they select any sub folder the ABC will go under it as D:\\MySub\\ABC
You did something similar and it worked but the difference is that above is trying to be installed under a sub folder as well.

http://forums.installaware.com/viewtopic.php?t=1894
If that can not be implemented as of now maybe the team of InstallWare can provide this kind of solution as well in the up coming update.
I would like to know if that is feasible?
Thanks,
Doron