Perhaps the SX-RK3 will work from SJS Cycles in the UK. I'm not understanding the specs from the website very clearly, but it sounds like it's compatible with a freehweel and a disk brake! Sounds good to me! A price of $188.33.
There's a TF-RS3 that is designed for trikes! Sounds like it takes a freewheel but no brake. Hmm, at $134.27, I think I'm willing to sacrifice the rear brake.
With all these hubs, I'll still need to purchase a new wheel though since the mag ones aren't hub interchangeable - or least I don't think they are.
CS-RF3 accepts a 8/9 speed cassette but is not disk-brake compatible. This is from the Sunrace blog. Same as the ideal CS-RK3, but it's not disk brake compatible.