Can you confirmed with your printer reseller that the Ricoh 8220 can subset duplex through Postscript only? If so, can he do it through the printer Postscript driver and prove it?
Once he has proven it, the idea is to redo the same test but while printing to a file. Then, we can look at the code and see if it is using the same commands as in the PPD (unfortunately, some printer uses commands which aren't in their PPD).
An other way to do this would be form you to set the whole job in duplex and add blank page in between form pages that need to be "simplex".