I think it would be fairly simple. They should 'sell' a one-way SWU cert for 10K or 15K. The difference between W and D for NA is 15K, half of that is 7.5K. The difference for Europe is 20K, can't remember off-hand what it is for OZ/Asia. Maybe they could have both NA and SWUs. These certs wouldn't guarantee an upgrade, they'd work just like the current status ones for the u/g window: Full Y at time of booking, SE 7 days in advance, E 4, P 2. Allow the general Aeroplan public to use these like a P. That way higher status members could still upgrade in advance.

I can't think of a simpler way to implement a points upgrade system.