However, in 2g, the water would have to be changed pretty regularly... and you'll have to heat it too, and that amount of water is hard to regulate.
Could you do a sandy beach type idea, sloping to one side, (like a frog tank) http://allaboutfrogs.org/info/housing/halftank.html
... and have aquatic plants and terrestrial plants? You could run a filter through that. I have my betta in a 10g, and the waterflow from my aquaclear filter doesn't seem to bother him.