All the while the company is trading the pension deficit remains a debt to the company ie BMI. If the company is sold the buyer would have to make good the deficit in much the same way as Lufthansa has been doing.
You can't just "dump" a pension on the PPF. This would only happen if the company was insolvent. In the case of insolvency all the companies assets would have to be liquidated in order to pay off it's debts, the pension fund being one of them. It would do Lufthansa no good to allow that to happen as they would get nothing!
If the company's assets were to be sold off piecemeal leaving the pension high and dry the Pension Regulator would have quite a bit to say and I don't think Lufthansa would like to receive all the bad press that would incur.
Lufthansa's first choice would be to sell the lot in one go (most unlikely) so IMHO I think the break-up option is the most likely outcome so as to be able to move mainline on as still trading and get something back for the slots. Willie says he didn't want Lingus because of the £400m pension deficit so I would guess he may feel the same about our pension situation. So who else would be interested I really don't know. A seriously difficult situation for Lufty hence Morgan Stanley and the new Chairman.