As a former moderator I can tell you that you become a moderator like so:
When Sync posts that he's looking for moderators, provide your e-mail in with your name and player name.
The following factors were surely considered:
Presence on the forum. Infractions if any (ideally none).
Previous bans from the game (hopefully none).
Fairly active in the game. Previous posts on the forum reasonably easy to read / helpful.
However since that early time much happened. Now there are only a couple volunteer moderators, and Sasp (the customer service rep) also helps moderate the forum.
As far as I know they aren't interested in adding more moderators at this time, or if they were they would probably just contact whomever they would consider for it directly.
To consider a banned player for moderator status seems to be extraordinarily unlikely.