Sorry, I was showing my ignorance earlier - Prologic's channels are left, right, centre and mono rear. So if they could be decoded within the digital realm (no idea if this is possible), it would be a simple case to reassign them to the respective channels within 5.1, dupe the mono surround to both rear channels, and slot in the LFE.
The levels might need a bit of fiddling, but really it's the decoding that would be the hard part. Any ideas?