Hmm, looks like a general policy to keep the 0xffff available as special value in case we ever need it, rather than a correctness requirement. I think though it's not worth the trouble changing all of those, because it entails checking each and every one to be absolutely sure.
About the recent gsm-tester patches: I'd rather have a is_int(min, max) function in osmo-gsm-tester and leave the values as-is. For that validator you can return a lambda like
def int_validator(min=0, max=65535): return lambda x: x >= min and x <= max
~N