You can often resell it for pretty good prices which helps pay for the cost of tearing it off.

That is a good question. First of all I would never suggest doing anything without the approval of your metal roofing manufacturer.

That said, as a manufacturer of metal roofing, I have been involved with a few successful installations over slate. What I suggest is using a self drilling self tapping stainless steel screw for fastening. If test fasteners show that your slate is such that these fasteners can be used without cracking the slate badly then this sort of installation has potential. If not, then tear off and if necessary re-deck will be required.