I travelled back from Spain yesterday with my US Citizen wife, travelling on my expired Green Card and extension letter. No issues in Madrid Airport, where the extension letter was recognised instantly by all airline staff (Iberia), and was through CBP at JFK in minutes...handed them my passport, green card and extension letter, and was asked no questions other than the usual customs questions as to any items we were bringing in.
Was really simple, and nothing to worry about either.