"of course direct air-to-air communication is frowned upon,"
Is it? I would never do it on an active manned ground station frequency, and never chat, but when using an airfield frequency, knowing there is no active manned ground, I would respond to any other aircraft.
eg ATC notammed closed for period, but taking off with out-of-hours indemnity, and passing military traffic tries to contact ATC. Respond that they are closed until xx.xx after no response to several calls.