Could these bcalls be of use for this?
FlashToRAM (http://wikiti.brandonw.net/index.php?title=83Plus:BCALLs:5017)
EraseFlashPage (http://wikiti.brandonw.net/index.php?title=83Plus:BCALLs:8084)
I managed to write a patch which writes zeros into flash and rename QuadReg to Q·dReg, so modifying it in RAM should be doable.
Writing back into flash after erasing the sector is probably also a matter of looping+stack?

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?

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

Edit: I figured, since the z80 is single-threaded, this (using interrupts) is probably the only solution

Sorry, back with another one 
A subroutine, which I will call inside my main game loop, should be executed with a delay of 1 second.
The subroutine may be called more than once in one execution of the main game loop, every loop will last ca. 0.01s.
Obviously, "Pause" doesn't work, because I want the main loop to continue running, and Pause seems to stop the entire program (the game should continue running).
My attempt at solving this was adding this timer to the main loop (I don't expect J to become larger than 256)
.Initially I and J are 0
I+1→I
If I=65535
 J+1→J
EndAnd then, every time I would want to call the subroutine, instead I would write the "time" (I,J) into the free space of a buffer (Buff(30)→GDB1)
(0th: I/256; 1st: I^256; 2nd: J)

So, for example, when I call the subroutine after 30min (180000s) of running time, "J" will be 2 (180000/65536) and "I" will be 48928 (180000^65536), which results in the timestamp:{191,32,2}. Because I want to execute it one second later, I will add 100 to the first value so I get {36,33,2}.
I don't expect more than 10 subroutine calls to "queue" at the same time, so the Buffer is of size 3*10, as I need 3 bytes of space to store the timestamp.
The main loop checks if there is a timestamp in the Buffer which equals the current time
For(A,0,10)
If ({GDB1+(A*3)}=I/256) and ({GDB1+(A*3)+1}=I^256) and ({GDB1+(A*3)+2}=J)
sub(DELAY)
0→{GDB1+(A*3)}:0→{GDB1+(A*3)+1}:0→{GDB1+(A*3)+2} .Clear space
End
EndAnyway, this is the only solution I could come up with.
This seems way too complicated for a simple task as delay, is there a better solution?

Very nice, the ternary solution is already so much cleaner!
If I understood right, a pointer-table would require me to write the adresses of all Pic variables into a Buffer first?