Great job NiLuJe!
Very similar idea is used in kukyakya's font hack. It'd been first released on last December. The first release had a file triggered script like porkupan's and fonts were in user partition like porkupan's, had an anti-brick scripts, and automatically using the respective original fonts if some fonts are missing from user partition font folder. Though fonts were copied, not mounted.
Anyway, it's a shame that it wasn't published here earlier.
I got the permission so, these are the links to his files. You might look at the repository for the source for runlevel3 implementation.
Note: It's not in English.
https://redmine.kukyakya.pe.kr/proje...iki/Customfont
https://redmine.kukyakya.pe.kr/proje...ont/repository
https://redmine.kukyakya.pe.kr/attac...omfont.r60.zip
Release60 is for 2.5 Kindle of course. This font hack has runlevel3 automatic disable switch which every Kindle hack should have.