As Kidd has pointed out if the A320 MPA gets off the drawing board it will be just carrying on a very long tradition that is almost as old as commercial aviation itself.
Another example is the Short Empire flying boat that were built as commercial flying boats but then pressed into service during WW2. In this case "
Coolangatta" was originally flown by Imperial Airways and then QANTAS before being pressed into service by the RAAF for anti-submarine patrols and for general transport duties.
It is also possible to go the other way, as demonstrated by the Short Sunderland flying boat that was originally built as a military flying boat patrol bomber but were also used during WW2 as unarmed civilian airliners. After WW2 a number of Sunderlands were converted for use by civilian commercial aviation, which were known as Short Sandringham; in this configuration, the type continued in airline operation until 1974.
An RAAF Sunderland Mk.III operating from Rose Bay Flying Boat Base, Sydney during WW2.
An Ansett Airline operated Short Sandringham operating from the same Rose Bay Flying Boat Base, Sydney decades after WW2.