Generate G-Code for Perimeter only

Here's what I'm trying to do...

Step 1: Lay down a few layers of filament with extruder 1.

Step 2: Come back with a cutting tool (extruder 2?) and and run it along the perimeter of the layers that I just had layed out.

How would I generate code for only the perimeter of the first layers that I layed down in step 1?

Ambitious. The software isn't designed for this but you might be able to slice twice and manually combine the gcode. Settings for the second slice would be:

Custom STL with the profiles offset in X and Y (adding material) by half the diameter of your cutter
No-extrusion gcode style
One perimeter, 0% infill
Microscopic but nonzero external perimeter extrusion width (Zero would set it to automatic)

You might have to add a G1 to some specific X and Y coordinates that put the cutter on the same side of the part as the perimeter start so the travel move at the beginning doesn't cross the part.