I'd just skip the undergravel.
If you plant the tank fairly heavily, the plants will do a lot of the filtration for you. The cannister filter would probably pick up the slack, and provide circulation. In the event that the cannister filter isn't enough, just use one (or both) of the powerheads with a sponge/prefilter attachment.
As well as being too deep for effective undergravel filtration, a lot of the substrates are also too fine-grained.
For setting it up in reverse, you would reverse the powerheads, so that they push the water down the tubes, and into the filter, and then up through the gravel.
If you really do want to use the UGF, maybe stick to plants like java fern, anubias, bolbitis, moss, etc., that attach to the hardscape. And floaters.