I don’t normally include the back band parts with the ultralight seat kits, but I can if you want. I think it would work fine, but I haven’t tried it, so I’m not 100% sure. Just send me an email if you’re thinking about ordering one and you want those parts.
Most of the weight savings in the ultralight seat kit comes from the seat bottom, which is separate from the backrest/back band.
(I’m assuming you’re talking about adding the ultralight seat and back band to a standard weight packraft – I would caution anyone against adding a back band to an Ultralight packraft kit.)