Brushless DC motor steady-state performance modeling
Brushless motors are more complicated in that it's the frequency of
  the input voltages that determine the speed/current/torque.


This is true for stepper motors and PM synchronous motors powered by AC, but PM BLDC motors usually have their commutation controlled by Hall sensors or back-emf zero crossings. So it is the speed/current/torque of the motor itself which determines the commutation frequency - same as in a brushed DC motor.  

The formulas for brushed and brushless DC motors are the same. 


  Most manufacturers, including hobby motors, provide data on motor
  constants, no load current and internal resistance, so I figure this
  must be possible.


Yes, however 'hobby' motor constants are often not very accurate, and using them can result in quite large differences between calculated and actual performance.