If I wanted to do that I'd probably make it a token hook and have zstart run my token hook on ram clear so that it is still persistant

As for writing flash, have you been able to verify that you actually unlocked flash?

Thanks for the tip, zStart is really useful. Unfortunately it seems to get deleted when doing an "All Memory" reset, however when adding it again, the "run on RAM Clear" option is still installed.
I checked Bit 2 of port 02h to confirm the flash is unlocked (http://wikiti.brandonw.net/index.php?title=83Plus:Ports:02)
I don't think I can avoid messing with the OS tough

From what I've gathered from this thread https://www.omnimaga.org/asm-language/(z80)-writing-to-flash/ it is possible to write into flash by finding a free sector to erase (I assume this is what zStart does, as there seem to be no side effects to the modification), then use WriteFlashUnsafe to write. When editing the original OS tough, don't I have to erase a sector that contains OS information?
I've tried reassembling the 2.43 Disassembly into an 8xu, but without success. I tried concatenating all pages, then compiling (SPASM tells me there are several redefinitions of labels and gives me a small file which, compiled with packxxu, gives me an unusuable 8xu). Compiling page for page of course resulted in reference errors.
(How) can one reasseble a 2.43 disassembly correctly?

From what I've gathered from this thread https://www.omnimaga.org/asm-language/(z80)-writing-to-flash/ it is possible to write into flash by finding a free sector to erase (I assume this is what zStart does, as there seem to be no side effects to the modification), then use WriteFlashUnsafe to write. [...]

Oh, right, the flash command won't accept any write comments unless you cleared hte page first.
So here's how to go ahead:
1. copy the page to ram
2. modify the copy in ram
3. erase the flash page
4. write from ram to flash

Be sure to test this in an emulator, you might break your calc hard-time otherwise

Writing back into flash after erasing the sector is probably also a matter of looping+stack?

Yeah, that should be the write routine you copypasted up there somewhere

However, I can't even get FlashToRAM to work. Since I want to copy page 1, I assume the source address is 0 and number of bytes is $8000. But I don't know where in RAM I should put the copy. Especially because RAM can only hold ~24000 bytes (sector size ~32000); I guess I'll have to use the archive?

If page 1 is currently in MemA (you can read up on memory mapping here) then it's a simple matter of
ld hl,$4000
ld de,someRamArea
ld bc,$4000
ldir