Not fourthed, I am having fun I am writing a program to insert all the tokens I need using BatLib I should have this ready sometime...

Awesome, welcome here It looks like you will need more peanuts o.o

Yeah, it was OS 1.03
Also, it does not work on 2.55MP This isn't looking good... At least with the 1.03, the font worked before crashing, but this time the font doesn't even display as changed before crashing.
Nevermind, I forgot to send the fontset Now it is just 1.12 and below that fail.

However, it does work on these so far:
OS2.30
OS2.40
OS2.41
OS2.43
EDIT: And 1.19. I will update as I test them.
1.18 good
1.17 good
1.16 good
1.15 good
1.14 good
1.13 good
1.12 Fail
1.10 Fail
1.03 Fail
2.53 Good except mathprint (doesn't crash though)
2.55 Good except mathprint

Well, I got an early birthday present from thepenguin77 so I have just now managed to confirm that it wil crash on OS1.03 and I still have to test others. It does work on 2.43 for sure since that is what i tested on before. I am right now working on a good fix, but to be universal will add a bunch of code

EDIT:(though that could be related to Bcalls)

When you set a font hook, it remains until you run a program that replaces it, disables it, or your RAM gets cleared

When using the Font Hook, you cannot do special things on your own. For example, you cannot use your own drawing routines to speed things up. About a year and a half ago, I introduced an 8x8 font hook in BatLib and this is something I had never seen elsewhere. I had to do some semi-hacky things to display fonts in a custom way as opposed to just changing the data displayed and I figured this might be useful for others to use. Pretty much, if you want to use your own routine to display the font when the OS wants to draw a char, try adding this to the end of your hook:
   ld hl,18
    add hl,sp
    ld sp,hl
    pop hl
    ld a,l
    out (6),a
    pop hl
    pop ix
    pop hl
    pop de
    pop bc
    pop af
    ret
I am not sure if that works across all OSes, but I don't have access to them all The main scary part that could cause a crash is the fact that this readjusts the stack. If other OSes makes other calls or pushes/pops, this will likely cause a crash.

I used that to let me draw the fonts directly to the LCD and make sure the OS didn't try to mess that up. Plus, it skips over tons of OS code You will need to handle inverted text on your own, though, too. I just check the flag and use CPL on the data if it is supposed to be inverted.
Screenie:

As you can see, my actual hook needs help, too, because menus don't work, but the main part is that 8x8 fonts can work
EDIT: I have tested some OSes:
1.03 fail
1.10 fail
1.12 fail
1.13 good
1.14 good
1.15 good
1.17 good
1.18 good
1.19 good
2.30 good
2.40 good
2.41 good
2.43 good
2.53 works except in MathPrint, but doesn't crash
2.55 works except in MathPrint, but doesn't crash

That's okay, once you make it to higher level math, you'll meet more people who love math

Here is an example!
round(((sqrt(5)+1)/2)^Ans/sqrt(5),0
That takes a value in Ans (should be an integer) and uses that to find the nth number. For example, 13 returns 233