With our sanctioned, dirt cheap unlock service around the corner, trying to jailbreak NoDo (without upgrade hacks) is a waste of time. I believe the ROI on time spent on hacking this interop limitation is much greater.
This limitation is implemented in PacmanInstaller.exe (on the phone); it scans the manifest for the flag and bails with HRESULT 0x81030120.
As Mango FFUs haven't been released yet, I haven't tested upgrade path 'hacks'; worse, this behavior doesn't appear to be reproducible in the emulator limiting current testing to those w/ Mango phones. (That should change in the next few weeks, hopefully.)
I'm interested to see what ideas you guys have!