Slowing down the spindle around corners?

As is generally the case with LinuxCNC, most things are possible with a little effort.

I assume that you are wanting to keep a constant feed-per-tooth value?

I see that there are two useful-looking HAL pins:
motion.requested-vel
motion.current-vel

I don't have a machine to try it on at this moment (I am at work but you could watch those two pins with Halmeter to see if they show what they sound like they should show.

If they do, then you could take their ratio (needs two HAL components, there is no HAL divide, you need to invert one with linuxcnc.org/docs/2.7/html/man/man9/invert.9.htmll and then use linuxcnc.org/docs/2.7/html/man/man9/scale.9.html) and feed that into a second scale component that modifies the spindle speed request before it is sent to the spindle drive.

In practice I think I would use a "lincurve" to determine the spindle override factor, the simplistic approach here would tend to stop the spindle at times when you didn't want it to. Also, you probably need to consider the effects of feed-override.