CO2 gas injection is used commercially and there are some residential systems available. Here is one of them:
http://www.poolsmith.com/
If you look at the FAQ's, you will get a lot of good info. It is sold to be used for pH control, and it appears that it actually doesn't control TA very much. In fact, they recommend that you use the system in a pool that has TA less than 160 for their system to be effective.