It just so happens that I bought a Spectra (the cheapest one, the 1200FF) new-old-stock just before Polaroid went bankrupt the last time. Spectra film wasn't very common here to begin with, and nobody has old stock anymore locally, but I do like the larger format. I'm certainly not opposed to square (I shoot 6x6 regularly), but with many people familiar with the 600 film, I find the Spectra format stands out more and "pops" a bit more while still having the familiar Polaroid edges.
Anyway, I sent Impossible a message and they have responded to say that the Image/Spectra version of UV+ will be available "within the end of the year". I want to take some pics with it around Christmas and New Year's, so that might work out, otherwise I'll definitely consider a 600 or better for me, an SX-70 with an ND filter.