FCC Registration Number Sign
FCC Tower Registration Number: The posting of this sign should take place no later than the time at which lighting of the structure is first required.
47 C.F.R. 17.4 (g) States that the structure registration numbers must be displayed. Your group should ask "is this readily visible to someone who doesn't have access to the site?" If a location is conspicuous enough to post a "No Trespassing" sign or the owner's name and logo, then it is likely an appropriate place to post the Registration Number.