Article 130 (1)(a) aerial work means any purpose (other than public transport) for which the aircraft is flown for valuable consideration if given or promised in respect of the flight or the purpose of the flight.
Unpaid flight instruction is a private flight and not aerial work.
FAR 61.3 (a) permits a US registered aircraft to be flown in a foreign country by the holder of a pilot certificate issued by that country.
Unremunerated instruction is therefore possible.
I do not think that you would get permission for aerial work (paid instruction) unless you had the appropriate ratings issued by the country of registration as stated in Art 21.
[This message has been edited by Noggin (edited 10 May 2001).]