Actually, linear interpolation does work (Adam - I actually took the time once and worked through all the equations and theory you mentioned, and it works out very nicely - at least for the drill motor, which is what I compared my theoretical results to).
Anyways, don't take my word for it - Joe Johnson posted a
spreadsheet a couple of years ago which asks you for a few simple parameters (voltage, stall torque, stall current, free speed, free current) and comes up with plots of speed, torque, power, and efficiency. It also has a sheet of information on FIRST motors (although be careful; the Fisher-Price motor is certainly out of date, and others may be as well).