I was snooping around the centroid forum and found this quote by cnckeith
"right now, 0-10 vdc and 0-5 vdc analog out is the only way to control a spindle motor with Acorn.[/FONT]
we are adding PWM out and the ability to use an axis output (step and direction) to Acorn CNC12 software which will be backward compatible with all Acorn hardware!. so yes with a future software update you will be able to use PWM or step and direction to command a spindle motor in addition to 0-10 vdc analog out...."
http://centroidcncforum.com/viewtopic.php?f=20&t=3310
So it looks as though acorn will easily be compatible for what I want (in the near future).
For the uccnc controller I think i would need to hook up the 4th axis to the spindle drive and have the mill spindle on a relay. In which case i would need to manually set the spindle speed? No idea if the software can work with that.
I really don't know much about linuxcnc but my guess is it can do it, probably better than uccnc.
EDIT- sorry don't know what happened to the font here