I suspect you'd probbaly be ok from a blood flow perspective, but you might not be very comfortable for long and there is the positioning question. One thing I've found really helps with SPD is sleeping on a much softer surface: our bed is soft, but still too firm - I find the couch works much better (and it's also easier to turn over). I think it must take the pressure off - when I sleep in our bed, I can literally feel and hear my pelvic crunching when I turn over. Now, I may well wear out our uphostery and do some kind of permanent back damage, but I'll take what I can get!
If that's not an option, maybe a couple layers of sleeping bag beneath you? Good luck!
Michelle, mama to Isabelle (03/04) and Tom (02/07)