Just to update this thread in case it is helpful for anyone, it looks like S2R/S4R/S4Rs have similar mounting points as the 2nd gen monsters. Although I do not have any firsthand knowledge, from my research it would seem that the rearsets are compatible, albeit with a few simple modifications.
The major differences rearsets between the generations are that the 2nd gen monster have mounting points for the passenger pegs directly on its rearsets and possibly a different length for the brake line. (See
http://www.ducatimonsterforum.org/index.php?topic=48449.0 & this apparently applies to Rizomas as well
http://www.ukmonster.co.uk/monster/showthread.php?t=42718).
If anyone has more specifics or firsthand knowledge, please chime in.