why would you grid tie with a 12v source circuit? its much more efficient to increase your source circuit voltage to some value higher that your grid tie inverter output.
Unless maybe you want battery backup.
The answer to your capacitor question is no. The capacitor will make your voltage rise higher based on the amount of ripple you have riding on your DC out of the rectifier. Im sure it will work to some minor extent, but probably not in the realm of where you want to be as far as a sizable power transfer goes.
If you are afraid of batteries, and want to grid tie, then get a grid tie inverter that does not require batteries and wind your hydro alternator for 250 - 300 volts.