The 'T' dollars can be kept at anytime,.. not just if you 'can't play that tourney'. There's a large handful of regulars in alot of the satellites on Fulltilt, who mostly play satellites but do not play any of the tourneys they've satellited into. You just 'unregister' and the 'T' dollars will show up in your account. If you win a satellite for a tourney you're already registered for,.. the 'T' dollars just automatically show up in your account.
As said above,.. .'T' dollars can only be used for SNGs, & MTTs,.. not cash tables. If you want to withdrawl your 'T' dollars, they need to be converted to regular cash in your account, at a rate of 5%.