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the yoke in the link above looks well designed for tandem use ? except you'd have to add some padding

As far as portageing using the seat frame for support, I do that all the time with my 3 solo boats.  MR (45#) and Bell (29#) standard bench seats - I use my pakframes on the front cross piece - haven't noticed any problems with the seat frames, having done so for several hundred portages ?

my Wen Rendezvous (about 62# as rigged) with a bench seat, I use the factory seat mounted yoke - it soes not cantilever like the link/pic you posted, but rather clamps, on and both the front and rear cross pieces are holding the weight of the boat.  If you have a bench seat in your Prism, then you know how Wen mounts them, using a metal flange for the seat drops - that flange isn't "solid" and the boat flexes every step down - i.e., every time my foot hits the ground, the boat flexes out and back in - kinda annoying, and I don't particlulary like that setup - likely it would be better on a composite boat with a stiffer hull ? (mine's Rx)

but in any case, the seat frame itself has held up fine, though that yoke's only been used maybe a hundred times ?  maybe 50 portages and the rest from the car to the river.

you could give the cantilever type a quick test just using a couple pieces of 2x4 or whatever - see how much flexing you get when applying that pressure in two different directions to the framework - that would be my concern, whether lifting that rear seat crosspiece was a good idea or not.

assuming that that round thwart in center is solid enough to lift and hold the boat by, you could rig a packframe setup for it (minimal cost at thrift store and a couple of 90deg corner brackets from home depot)

or alternatively, have a backpack adjusted so that the top of the pack is high and just rest the round thwart on top of a lightly loaded pack ?

or have a transverse groove routed into the top of your head, and fit the round thwart into that for a true canoe hat?