I've read that I can use a 10spd Shimano road shifter to operate a 9spd rapid rise rear derailleur. Is the Tiagra flat-bar shifter the one I'm looking? Or if there are others, which would be best for a mountain bike?

What's the point of this? Is it that you can't find new 9spd MTB shifters?

Perhaps a ShiftMate could solve that. Or you could buy second hand.

+1

OP, what do you mean by "road shifter"? Are you talking about STI shifters? Just get a 9 speed, it will work perfectly and there are plenty of second-hand, cheap pieces around.

Also, if you do go the ShiftMate with a 10spd shifter, it might be better to fit it near the shifter rather than the derailleur, in order maximise signal-to-noise in the system; the cable pull per 10spd shift is pretty damn small.

Of course, you'd miss out on the removal of a bit of friction from the RD loop the ShiftMate provides, and it'd look a bit weird, so try it the normal way first.

The point is I need a 10 spd shifter because I have a 10 spd drivetrain.

Then I'd say just get a 10spd MTB shifter and ditch the Rapid Rise RD.

I have a feeling the cable pull increments aren't exactly linear, so a backwards derailleur sounds like a headache.

I don't think I can do that. I'm too attached to low-normal shifting :|

Oh well, I hope my guess about the non-linearity is wrong... afraid I can't be of much help with that.

Shouldn't be necessary. I'm using a Shiftmate to allow Campy brifters to shift a 10-speed Shimano rd and cassette and it works perfectly mounted at the rear derailleur.

But if it didn't hold its tune, that'd be something to try.

Anyway, I think the Campy brifters pull more cable per click than Shimano, no?

Rapid rise derailleurs will work perfectly in any application where non rapid-rise will work, IME. Meaning, the rapidrise 9 speed should work with the 10 speed flat bar road shifters.

I don't know about the front derailleurs, though.