So here it is:
From the Inkscape-generated G-code file, it's now possible to selection a region of the code and "export" it as a section. Before actually running the g-code, the VB6 app adds in the extremities, so when it comes to routing anything, we can double-check that it will fit inside the boundaries of the piece being milled.
The M0 command after each "preview move" halts program execution and allows us to take as long as necessary to check the position of the cutting head. The pause in the video is simply because I had to power up the spindle manually (since it can't yet take spin-start commands from Mach3).
The routed section exactly matches the extreme positions of the shape as plotted at the very beginning. After routing the entire guitar fingboard, we ended up with something like this....
Routing hardwoods doesn't half leave a mess on the cutting bed!