I know this is a crazy idea...
Anyone know how feasable it would be to swap a 3rd gen rear suspension into a 5th gen?
I understand it would require a lot of doing...anyone think it's worth it? for the irs and bbk option?

 

I'd say you're better off doing custom than trying to do a swap. The biggest issue you're going to face is the rear struts/shocks. The strut towers in a 3 gen are very tall- they go to near the top of the back seat.
pic for reference: http://blehmco.com/imagens/rstb/rstb4.jpg

Thus, you'd have to either rebuild the entire rear unibody of the car, or figure out a custom spring/shock combination that would work.
You would also have to find a way to make the 3 gen rear subframe work on the car, and that would be no easy task. By the time you've done all of that, you might as well just do a complete custom suspension back there and rid yourself of the weak links on the 3 gen rear suspension as well.
(The 3 gen suspension is better than the 5 gen, but there are many other designs out there that improve on the 3gen as well.) Might look at the 02-03 Altima rear suspension design, or the 6th gen.
Both of those use a rear drum-type parking brake which is the same size as the 300ZX and maybe the 350Z rear drum stuff. Thus using a larger rear disc is as simple as finding a setup that works on one of those cars and making a caliper relocation bracket. no parking brake hassles to deal with.