O my...,
Skinnyhouse, I while reading your answer I got an horrible thought...

Stitching is nice when downloading, but for people participating and/or who want to export their custom fonts to GC (here comes the horrible thought) > why not doing a xcf "splitter" as well?
Something like splitting the layers stack in 3 or 4 or more parts/groups, then "auto" export each part/group as same_name[].xcf, thus ready to upload in a click
Maybe all in the same script.py with an option like > "do you want to split" or "do you want to stitch"?

That was just a though

In all case huge thank you for everything you're doing for Custom fonts

Thanks so much Skinnyhouse, I downloaded & installed it without any error.

Link to version 02

Version 02 download