I just did a little research for all of you, who don´t want to keep your custom font on your MS. You can copy them to flash1 on EVERY firmwareversion. Just take your ltn0.pgf, and make 15 copies of it. Rename them so that in the end you have ltn0.pgf, ltn1.pgf, ltn2.pgf, ..., ltn15.pgf. Make another two copies which have to be renamed to kr0.pgf and jpn0.pgf
You should have 18 files (ltn0 - ltn15, kr0, jpn0) which should be all the same except the filename. Launch your PSP and start FileAssistant 0.4. Navigate to Flash1 and create a folder named modf. Dump your registry and change font path to flash0:/modf. Fix it using fixupdreg3. Get Dreg and Ireg back to registry path and restart PSP.
Advantages: No space on MS needed for custom font, PSP seems to run faster
Disadvantages: I couldn´t discover any.