With Faalavau, it's my understanding a Super Rugby contract can overrule a 7s contract. Or at least the 7s program is happy to let a guy go to pursue his dream..
They don't technically overrule, a super rugby player is still contracted to the ARU as is a 7's player, which means its easy to process the transfer between the two..
Historically the 7's contracts have been financially inferior to super rugby contracts, and there is also a higher level of 'prestige/accomplishment' with XV's.
This is why most players opt for Super Rugby contracts and the 7's program has been happy to accept that as a natural course..
In due course we will likely see this diminish, the Aussie 7's side has a sponsor for the first time(QANTAS) and the 7's tournament is also earning more from sponsorship and broadcast rights which will all flow into the players income. With the Sevens RWC and then the Olympics approaching the 7's coaches will become more reluctant to release players for Super Rugby in the future as well.