I like what Nun is doing with the compression sack threaded thru the saddle loops. I've used smaller compression bags threaded directly to a rear rack on some UL trips, and they worked well. I didn't bother to take them off the rack, just left them on the bike like mini panniers, and loaded and compressed with the bags still on the bike.
this isn't a closeup and i apologize, but the compression bags are red, small, and one on either side of the rear rack. I got them at a MSR wharehouse sale for a dollar apiece, and they are very stout nylon.
for anyone crazy enough to give UL packing methods a try, the method of threading a compression sack to a seatpost and seat looks very viable, and similar to what I was envisioning as an easy modification that could be made to a compression sack.
EDIT: whoops, got a framebag now to go with my gastank and home-sourced feedbag, and should have a seatbag en route shortly. will construct a front bag, and give er' a go.