One possibility, as you appear to be using software step-generation, would be to combine the output of the normal A-axis stepgen with the output of a velocity-mode stepgen.
Code:
loadrt stepgen step_type=0 ctrl_type=ppppv
loadrt or2
addf or2.0 base-thread
net a-pos-step stepgen.3.step => or2.0.in0
net a-vel-step stepgen.4.step => or2.0.in0
net a-both-step or2.0.out => parport.0.pin-99-out
net axis.3.motor-pos-cmd => stapgen.3.position-cmd
net motion.spindle-speed-out-rps-abs => stepgen.4.velocity-cmd
Would this be the only spindle, or do you already have motion.spindle.... operating a tool spindle? If that is the case then you would have to command the A-axis spindle with M68 commands (or switch to the development branch of LinuxCNC which supports multiple spindles)
