RE: address patterns as 0-255:
Say we have the address 0-255 stored in A, this will retrieve the value on location A:
PN / A 64 % A 64
And this will store 7 in location A:
PN / A 64 % A 64 7
But I guess you already figured that out 
EDIT: It’s not my itch, but if you’re ready to roll up your sleeves, this should be fairly easy to add as a new OP, could be useful to others. PX seems available, so the above could be:
PX A
PX A 7
The OP should IMO handle addresses outside the 0-255 range (including negative numbers) elegantly by wrapping them into 0-255, so the following should yield identical results:
PX 0
PX 256
PX -256
Or you could step it up a notch and have it accept user defined wrapping points:
PX.WRAP X (turn wrapping on/off default to on)
PX.MIN (lower bound, default to 0)
PX.MAX (upper bound, default to 255)
So to work with only locations 10-19 in pattern 1 () you could do
PX.MIN 74; PX.MAX 83
PX 0 (same as PN 1 10)
PX 1 (same as PN 1 11)
PX 10 (same as PN 1 10)
NB: Hope I got the wrapping right, but you get the idea…
I guess further discussion related to such new OP should go in Teletype 3.2+ feature requests and discussions