The AMA rules that apply:

AMA General:
"4. Identification: All models, except Indoor, will be identified with the owner's name and address or AMA number, on or in the model. Individual events listed within this (Competition Regulations) may have additional identification requirements which must be adhered to."

CL General is silent regarding the AMA number, and the CL Racing rules specify for the supplemental events of Class I and Class II Scale Racing (i.e., 1/2A Scale racing), where they say:

"The model must have racing numbers on the fuselage sides and the upper left-hand wing. The contestant’s AMA number should appear as part of the scale like license number on the upper right-hand wing. It may be preceded by “N’’ at the contestant’s option."

And for Sale Racing:
"2.3.3. The contestant’s AMA number should appear as part of the scale like license number on the upper right wing. It may be preceded by “N’’ at the contestant’s option."

The NCLRA rules for B-TR are silent regarding AMA numbers.

Bottom line IMHO is: as long as your number is on or in the model, you're legal. Now, also IMHO, if it can't be easily viewed and read from the  outside of the model, I feel it wouldn't comply with the AMA General rule. Edit to add: "wouldn't comply" because if you can't see the number, you can't verify that it's there.

Most simply put the AMA number on the upper surface of the outboard wing.

Regards,

Bill Lee