There are 127 possibble continuous controller (CC) messages. Only a fraction are used (to the best of my knowledge). Each CC has 127 (maybe 128) possible values. So you would only need to dedicate 2 or 3. I think they can afford that.
As for the tempo idea, I take it back. Being that there are only 127 possible values per CC, and there is double that amount in tempo values, it's not really possible unless you limit the controll to every other BPM (eg. 128, 130, 132, etc.).

By the way, I don't claim to be a thorough guru, and it's possible that I may be wrong about my understanding of CC messages.

That disclaimer stated, I think I'm right :-)

One more thing: I noticed in the midi implmentation, there are sysex messages to select styles in song mode. So apparently it's not so impractical to implement-it's sort of already there. All they have to do is allow you to control it in real time.

-Michael