I pulled the spacebar out of my set, and it does appear that the odd stabilizers are costar-compatible. The stem on the spacebar is in the wrong spot, though, so I can't test it for certain. The "bar" on the stabilizer does indeed fit (though tightly) in the costar stabilizer slot. I had to pull the old stabilizer out for obvious reasons, though.
I may mod the stem, as the cherry PBT spacebar is excellent.
Edit: Some sanding may be required, as the cherry MY stabilizer bar seems a bit thicker than the costar. Also, it might not work at all, I wasn't able to test the key travel/action with the stem unmodified.