Unfortunately there’s no tweak for this. XPmode is designed to be a single user crutch to get older apps to run. So there’s no easy way to patch it. I email the person that has it on their machine and on the weekend I patch, I tell them to turn it on and leave it on so my patching tools can reach it.
I personally find that most apps can run just fine in Win7. If they put up a fuss, try editing the regestry location and also their folder location and open up the permissions.
You may find with older apps you need to do this — After you turn on User Account Control in Windows Vista, programs may be unable to access some network locations http://support.microsoft.com/kb/937624
Sometimes you have to make the choice for the business and get the app running.
Chad blogged about it here.