I think the least intrusive method of adding a way for f2p to earn coins from playing would be to add GW2 gem / gold market in addition to what bioware has currently proposed.
In it, you can buy gems (cartel coins) for gold from other players.
Some subscribers will not want 1000 minipets or 100 mounts. So their 200 cartel coin allowance will seem pointless to them once they have bought the few things they want. This would allow them to make quick credits selling their allowance on the market.
Prices adjust automatically to demand, there's even a stock ticker in game showing the highs and lows of the gem / gold market in gw2.
This could be implemented into swtor as a section of the cash shop UI.
This would allow f2p players the ability to access content through playing. Why do that? Watch this. http://www.youtube.com/watch?v=AWMBeLrZoyw
It would also add an additional benefit to being a subscriber. Don't like dailies? Fine, sell your CCs. Can't buy a perk with CC? Fine, sell your CCs for credits to unlock it.
Another additional benefit would be lower queue times for WZs, FPs, and OPs because more people would be able to partake. Subscribers, would of course, be places in the front of the line.
Almost every f2p model includes the ability to gain access to cash content through playing. This would be one way swtor could do it that would promote active play, and thus a more lively world, without touching the structure of their game much.