I've also seen a workaround to expose Instax film in a Rolleiflex or other TLR with a polaroid back, and polaroidconversions.com offers conversions of Polaroid cameras to use Instax films.
To my mind the bigger benefit rather than a better lens is regaining manual exposure control. I enjoy instant prints and use Instax wide and mini (with an SP-1 printer) quite a bit. The "problem" with the Instax cameras is they're fixed focus, fixed aperture (f/14 at least in the case of my Instax 210), and automatic flash. Personally I'd love a manual-exposure Instax camera that has facility for on and off camera flash synchronization.