There are 36 bytes of RAM from 62982 to 63017 which can effectively be rendered invisible if an optional ROM is not installed. Whatever you poke into those 36 bytes will stay there regardless of any BASIC or M/L programs that you may run.
The original routine from 62981 to 63011 checks for the existence of an optional ROM every time you power up, if one is present the value 255 is stored at 63018 and the name of that ROM is stored from 64164 to 64171. Addresses 63012 to 63017 are used when selecting the optional ROM from the Menu.
POKE 62981,201: RET, which effectively makes your 100 think that an optional ROM is installed.
POKE 63018,255: To prevent a cold-start on power-up, the remaining 36 bytes are now free to poke in anything you want.
Interested in more free space to poke stuff into? TELCOM uses 64704 to 65023 as a buffer for its previous screen function, it is also used by the Lucid ROM when it is installed and for some Menu functions which begin at 64904. There are 320 bytes, the first 200 of which are perfectly safe. The remaining 120 bytes may be used as long as the program using that area doesn't use the Menu's date and time routine while running.
Addresses 63109 to 63362 are reserved for the INPUT buffer, the later part of which has been used to store short M/L routines while leaving enough space for normal buffer operations.
The MDM & COM buffer from 65350 to 65413 can be used as long as it is not being accessed for input or output operations, there are 64 more bytes.
Address 65451 returns a non ASCII value for the last key pressed and it is also first address in the type-ahead buffer which uses the odd addresses from 65451 to 65493.