If you don't want to use a rack that mounts to a QR (The disadvantage of a QR-mounted rear rack is that taking the tire out is more complicated) you can try to find in a hardware store what is known as a "p clamp" which has a cross section the shape of a P and the end has a threaded hole through it. You migth find them in the plumbing sections but they may allow you to put around the seat stay and provide a bolt hole for you to put a regular rear rack mounted to the P clamp around your seat (or chain) stay.

Am I reading you right? I am a bit confused by what you mean by seatpost rack, are you talking about those racks that just clamp to a seat post with no bottom support and a low weight limit?

If you're talking about modifying a seatpost rack to keep the rack from swinging around, do you have V-brake bosses on the seat stays? If so, you can get a 90degree angle bracket and perhaps a longer hex bolt and put the 90deg bracket behind the v-brake cantilever and then use the longer hex bolt through all. You might not even need the longer hex bolt! Now, once you have the angle bracket on, all you need to do is rig up some support from that to your seatpost rack and you're set. The only disadvantage with this, is, depending on the angle bracket you use, it might hinder the parallel arms on the v-brake so you might not be able to open the pads as far as normal which might interfere with removing a full MTB knobby tire when not flat.

I have a OMM rack on a bike with lower eyelets but no upper eyelets so I use this method for the upper support on my rack.

Hope this helps!

Jay