To answer my own question, Boingo was one of the login options on my flight (A380) but my Boingo account details from Amex weren't accepted. Seemed to imply that the Amex subscription doesn't include onboard wifi, but I'm not sure about that. By contrast the Mastercard page expressly says it does, but I can't say if that would've worked or not.

Anyway, I bought the 4.95 USD 30mb voucher for the flight instead and that was enough to get me through emails for most of the flight, so worked out ok for me.