I think that longer format was imagining a more traditional sporting league with all teams traveling around playing home and away. Whereas for the RPL the games will be 23-30 minutes in total duration, but it'll still be all teams playing at the same venue. They wouldn't want to go too high with 13 players per squad.
I wonder if a similar concept to this would work here, but with both men's and women's teams. With 6 teams and both genders you could keep the standard 14 minute matches and if each team plays once per night it'd take approx 2 hours. Could be a nice tv product for 11 days in November, or during the summer, featuring most of the best sevens players from Australia, NZ and the islands, and maybe the odd super rugby player. If you played it all at 1-2 smallish stadiums in a single city you'd keep costs down.