My thoughts:

Yes, please, in core would be just fine .
As far as complexity goes: A value, and a calculation method (add or multiply) per payment method would be nice.

I realy like the "one page checkout" Ubercart provides. But digging into this matter I can't help to think that it would be a lot easier to have more steps like :cart>Shipping>paymentmethod>adress+checkout. It would make things like this a lot easier.. but he, the easy way isn't always the best.

Note: I'm a bit concerned about the Javascript reliance of the method I used.

I would love to make it work, I don't have time for it the next two weeks though...

Kees