I've found evidence that Senoue composed Bonus Stage (Slot Machine) (a part of Rings and Diamonds Land) through the second (and now only) unused track from Sonic 3D Blast.
http://www.youtube.com/watch?v=vvTGPGDMwBw - Unused Bonus Stage
As you can hear, it includes the start of the game's Special Stage (which is definitely done by Senoue) (
http://forums.sonicretro.org/index.php?showtopic=6464) and the start of Slot Machine before it loops. This officially makes Senoue the composer of all three bonus stages.
Also, for Sky Sanctuary, which in the group of Sawada, Kashima and Drossin (
http://www.geocities.co.jp/Hollywood/4764/gmcd/cd74.htm) (
http://forums.sonicretro.org/index.p...755&start=1755)
->Drossin proves that Sky Sanctuary is not his composition via City in the Clouds (
http://www.youtube.com/watch?v=2stvbSJk4PA)
->Kashima doesn't list it on his site; only Special Stage is there, implying that that was his sole contribution for the game (
http://web.archive.org/web/200105051...mmr/booth.html)
Therefore Sky Sanctuary can only be Sawada's.
This song [a Sawada definite] should also be a good indicator of this.