A secondhand seat is $150. Me not working, and the XJ killed what I had left of my, err, buffer...
So anyhow, I actually used to do upholstery once upon a time...if there was enough market for fiddling with bike seats, I'd probably consider doing this a bit more seriously...
My conclusions thus far, are that adding hard-points, while attractive from a design and functional point of view, is not exactly practical in my current circumstances. I don't have access to a welder, and the only person I trust to do this the way I want (dad) doesn't have one either. Apart from that it is a top idea, and I probably will end up doing it this way at some point, if only to prove a point.
So, to pass the conversion, I think a fibre-glass shell over the rearmost section of foam will suffice. Perhaps with a bit of steel reinforcement to that I can carry my saddlebags with impunity. Hmm...and some bi-directional weave glass...I know just the place to get some offcuts.
I can remove the foam, and cut a hole in the plastic section of the seat, which should allow me enough room to store my wets and/or a bunch of other crap. Importantly, I will not have to do anything to the vinyl cover, which means the whole process is reversible if I change my mind later on.
The only real issue with making a new pattern for the cover is I don't have access to a vinyl welder to ensure the covering remains water tight. I am looking at getting one, and a guy in japan just told me they nearly gave some second hand ones away

Dang it!