You might get away with creating a Room Number 155 (occupying the memory range #5B00-5BFF), if you didn't have any guardians in the room (i.e. just an 'FF' terminator byte at #5BF0, followed by 15 NOPped out bytes from #5BF1-5BFF).
The bottom end of that 'page' of memory is the beginning of the stack, so Return addresses from CALL commands and register-pair values placed there by PUSH commands would occupy the area where the Guardian Specifications would otherwise sit. Depending how much the stack is utilised during the game*, the rest of the room's data might be left alone? (The stack entries would get copied to the end of the Room Buffer upon Willy's entry to Room 155, but with no adverse effect on the running of the game if they are preceded by the 'FF' Guardian List Terminator byte.)
However, I don't think that any items could be displayed in a room in the range 128-255, unless the Draw the Items code were to undergo further modification. As things stand, Bit 7 of the first byte for each item's definition stores part of its y-coordinate, which is discounted (reset) for the purposes of comparing each item's room number with the current room number at #93DF.
* Obviously the stack as an entity is used a lot, but if values get PUSHED on and then POPPED back off, then you don't progress 'up' the stack as a result; so what I really mean is how many consecutive entries (which fill up two bytes each time) get put there, starting at the bottom (#FBFE-FBFF) - i.e. consecutive PUSH commands, sub-routines called from sub-routines (generating multiple RETURN addresses on the stack), that kind of thing. Any more than six of those, on top of the two-byte entry which defines the game's starting point (namely #869F, or #87CA if the 'code entry routines' have been expunged), and you'll reach the location of (and overwrite) the room's Guardian Terminator byte at #5BF0. Which could lead to corruption of the whole game file, a la 'The Attic Bug'!
Perhaps one were to insert the value 'FF' at #5BF0 in a test file, and then PEEKed at the value of that byte during the course of a game, then one could verify that it isn't ever modified from its initial value throughout the game, and so anything from that point upwards in 'memory page #FB' could safely be used to define a Room 155? But for the sake of a single room with no guardians and no items, there's probably no point other than as an 'academic exercise'!?
EDIT: The memory range #5B00-5BFF is also apparently used as the 'Printer Buffer' - I presume that harks back to days of yore when you physically plugged a Spectrum into a Dot Matrix Printer or suchlike, and that alternative function wouldn't affect the values in that range, in this context?
Edited by IRF, 07 September 2016 - 02:26 PM.