Most whisper users are getting 2300-2400fps from 125gr BT from their 20" barrels. Your 30PPC sounds like it should work well as the reports given from hunters using this bullets at whisper speeds has been very good. Time permitting I'd like to experiment some more with these bullets.
Well bigfoot since no one has any information to help looks like you might just have to go right ahead and make one. Failing that you could take the proven quiet route and try a 300 whisper. You still should push your 125gr BT at 2,400 fps from that tiny little case.
I'm not surprised that this post might sit here awhile, not many people have experience using pistol powders in a rifle cartridge like the "Quieter Coyote Rifle" article did. I read the reloading threads in the Whisper section and everybody seems to be using the fast rifle powders even for subsonic.
Supressers are a good thing.
Not only do they save your ears but, in prone, they keep the dust out of your face. They are also flash suppressors in low light.