Hey- good job of making the most of a bad situation.
I tend to use this sort of thing as a bit of an acid-test for stores, if they sell something like dracena, pothos, 'peace lily', or some other sort of terrestrial/bog plant as 'completely aquatic', I'm very unlikely to buy anything from them.
As to substrate; I think pretty much anything would work, with the plastic canvas basket you made, gravel would be great, cheap, and easy. If you want something finer like soil or something, consider using some sort of porous/permeable holder like a terracotta pot, coir(well rinsed) basket/pocket, or really fine mesh (pantyhose would be pretty cheap...).
Also, check out various forums/websites for info on paludariums/ripariums, as they tend to deal with a lot of bog plants, and growing things with the leaves in the air and the roots in the water. I've been out of the loop for a bit, but from what I remember, I think the trend is towards chemicaly inert, larger-particle substrates (gravel, hydroton, etc.). Most of these plants are seen growing in vases on people's office desks, just getting an occasional top-up of tap water(and it seems that many of these (dracena, pothos, etc.)can live for years at a time in these conditions), so pretty much anything in an aquarium/riparium setting would be orders of magnitude better for the plant.
For a lot of paludarium/riparium set-ups, having emersed plants is pretty much the de-facto filtration method- the plants don't seem to need the CO2 infusions, or even as much fertilization/supplements as the fully aquatics, and can strip the ammonia from the water about as quickly as it is produced.
Oh- also, it looks like you may have gone overkill on the zipties. I think they are great for applications like this, but if you don't have an excess of them, just using them at the corners, or every couple of inches or so would probably work pretty well. It may be a bit more effort, but fishing line, or possibly even polyester thread could be used to 'sew' the baskets shut on their seems. though, this probably won't matter unless you are producing these on some sort of large scale...