One option is to pay it off, exactly as Lufthansa did and get on with the business of being an airline.
BA have the cash (yes, cash!!) to do it and have been paying off debt at the rate of £1.2 billion per year for the past five years whilst still making a profit. Why should a pension debt be any different? It is deferred pay after all.
See the 11 pages of
BA pilots 'prepared to strike' thread on R & N.