I believe Mach will feed as slowly (and smoothly) as you want, provided the resolution of your motor tuning is fine enough.
I temporarily set my steps per very high and added 8 decimal places to the axis dro, then commanded f .0005" feedrate and can see the dro incrementing normally.
It's all math and pulses, so I'd say you're fine.
Highest resolution I can actually achieve, direct coupled is 50,000 ppr x the pitch of the screw.
I toyed with the idea once to make a stage similar to yours using a micrometer head coupled to the 50,000 ppr motor.
40 tpi x 50,000 = 2,000,000 steps per inch.
This is what I used for simulation.
Russ