I have a two post in my garage and have thought how do the dealers lift the 3wheeler.
These days most of the dealers have two post lifts. Therefore I think they must use a lifting beam that goes between two of the lift pick up points. It would have to have some shaped parts at the ends to fit on the lift pads and probably at the lifting point for the car but nothing that a local steel fabricator could not weld up for £100 to £150.
On the other hand a four post lift will have no problem with the front wheels but you would want some load bearing center panels for the rear wheel again that needs done by a fabricator and load tested.
This thread never came up with a great answer.
http://www.talkmorgan.com/ubbthreads.php/topics/256045/1Talk to a Morgan dealer that must have been thought about and a safe working procedure put in place to lift the cars.