Someone else would probably better at answering this. But submissions to the
livery database must be GPLv2+, so it is legally possible. However I think the general idea is that FGAddon will only hold a small subset of high quality liveries, and the rest remain in the livery database. I heard somewhere, though it might be a rumour, that there might be plans in the future to allow for liveries from the database to be downloaded in-sim, similar to James' recent work allowing whole aircraft to be downloaded in-sim.
Regards,
Edward