Jan Olieslagers:The weight limit is there to minimise damage on the ground in case of a crash. Only they didn't think of limiting the speed, too... The current ultralights are way more performant than anything the then regulators ever thought of!
Frankly, what wing loading has to do with it eludes me; except if such a rule would automatically bring thick slow wings.
I think the first part of your post demonstrates exactly why limiting wing loading would be a good way to define a microlite.
As you say the weight limit was there to minmise damage in the event of a crash. In other words limit the amount of energy involved. The more modern ultralites/microlites (choose either depending on local definitions) have a lot more energy due to their speed.
By limiting wing loading you limit energy. It's difficult to have low wing loading and high speed.
I rather suspect the weight limit was a crude way of limiting wing loading, so why not actually use wing loading instead of weight.
Perhaps there should be an upper limit on stall speed and a max level flight speed. Say less than 25 KIAS stall and 75 KIAS max level flight.