Hi Chris
new to the forum, but thought that would chip in so to speak, I too have a HMC160 and I was wondering the same thoughts as your self re the 4th axis, I have at present a 4th axis fitted but not using the inbuilt 4th axis that rotates at 45 deg intervals...it is borrowed from a fried and this is a true rotary table with stepper motor fitted, same as the one from arceuro trade..

My thoughts are if the original 4th axis has a reasonable worm drive then by rights it should hold the pressure of the milling operation, however the play on the worm wheel is the main problem so to eliminate the play and then use the stepper to hold the table in theory should work...
I see what you mean about the brake as the original forth axis goes past the 45 deg point and then reverses to hold the table in place, thoughts on this is the table in effect is being held against the stops by the stepper motor and this reduces the play on the worm wheel and the table is rock solid so the original motor is holding the table in one direction against the cutting forces, and the stop in the other direction... Hence my theory if the play could be removed from the worm wheel then in theory it should hold the table at any angle....

I have looked at this may time and had the same thoughts just time on my side that is a little restrictive.

On HMC160 have also fitted 3 phase motor and inverter, plus using mach 3 and spindle feedback, still using the original sd stepper drives for all axis with an interface to the pc that I have built...

Good machine done loads of machining and works a treat...
Best regards
Paul