I'm thinking of converting my lathe to Mach4. The bed is pretty worn out and I'm wondering if the surface mapping functionality could be used in the X axis to compensate. It seems to do exactly what's needed - shift one axis by a certain amount based on position of another, but there doesn't seem to be a way to set it up in the X axis or in lathe mode. I know it's probably better to start off with a better machine, but it weighs 6000+lbs and was extremely difficult and costly to move into position.

I'm not familiar with Mach 4 but I guess the surface mapping is intended specifically to correct for a non-level bed for a mill or router, so the function may not have been written in a way that would apply to a lathe.  In any case, searching around there are a couple of threads implying that it doesn't work very well anyway so people tend to use a separate program to map the surface deviations in Z depending on W and Y, then correct the g-code before loading it into the machine.  That might be possible on a lathe but sounds harder.

A couple of thoughts that may help though.  In what way is the bed worn, is it mainly in flatness or does the carriage also/instead have lateral play due to excess wear near the chuck?

Have you thought of correcting the bed?  There is a material which IIRC is called Turcite, which can be used to fill worn areas of machine slides "in situ" to deal with wear without needing to move the machine.  I think people have also had some success just mixing graphite powder with epoxy resin to form a putty.  You would probably have to take the carriage off but at least not get the machine moved to a grinding shop!  Have a search around for this.

Do you intend to replace the feed screws with ballscrews?  Obviously if you could correct wear using a deviation table you need to know exactly where the carriage is, and backlash (made worse if the screws are worn) would make this more difficult.  I know that Mach3 has BL compensation, and probably M4 too, but in my experience this doesn't work very well with largish backlash.  On my lathe (Super 7) I have a ballscrew on the cross slide but keep the standard leadscrew.  As most lathe work is over a small Z range near the chuck you could find that just doing something similar, and taking up as much lateral play as you can with the carriage gibs, would be good enough for most jobs.