I would use an aluminum plate instead of plywood, and make sure the plate goes a few inches past each of TWO rafters, whether your rafters are wood or metal.
It will probably hold your weight, at least for a while, but since your roof is not arched, any way that you hang a swing is likely to cause the roof to dish inward and create leaks where roof meets walls or in roof seams.
Really your best bet is to get a knowledgeable builder to do a bit of disassembly -- like taking off part of the ceiling -- to see exactly what the structure is, and probably add to it.
Edit: For example, adding an edge-laid 2x4 or 2x6 or similar aluminum beam, recessed into the ceiling, and posts recessed into the walls, could make the swing actually safe to use :).
I understand what you mean about how deciding you do not care about resale value frees up a lot of options!
