The first issue I'd have is the under-gravel filtration on that model. Under-gravel systems are bad for planting.
So here's a thought. Get a box of Laterite substrate and don't use the under-gravel system in the tank but DO use the lift-pipe and pump. Get a pre-filter sponge that will fit over that lift pipe's bottom end. Sponge-filter sufficient for a betta - done. Looking at the pump and pipe I'd assume that the Fluval pre-filter for the Edge series should be fine. A different option would be to get a black cube sponge and carve it in the shape of a rock then stick the pipe down into it.
The laterite will support plants just fine and its a nice red-brown color.
For use with a sponge, turn the motor flow down as far as it will go.
As far as seeding goes, just put the assembled sponge-pipe-motor in your main tank and stir up some debris while it runs, about 24 hours should get it good and happy.
If you feel like it, this solution you could cut the plastic pipe to a length just long enough to support the sponge and have just a little black motor-and-sponge assembly hanging in the back.
Just for the visual reference: http://ak.buy.com/PI/0/500/203466003.jpg