I agree that plants are a good step in the right direction but you won't have enough for complete balance... your snake will definitely be able to make a gross overload of waste. Think of the food pyramid... which can be matched to the ecosystem pyramid. How much plants do you need to support one snake? It's more like an 1/2 an acre, not a square metre or two. That, and you can't have a closed system... because you are always inputting. You feed your snake and that prey has to go somewhere after the snake is done with it. Plants will grow, but not enough to compensate in a small enclosure.

You are putting a lot of time and effort into this... I would strongly encourage a flow-through enclosure. Drill a drain into the bottom. Water goes in through the top via misting, water fall, whatever, picks up wastes like ammonia and carries it out the bottom. You have the choice to use fresh water as an input, or filter the waste water and recycle it. Either way, an external filter allows larger filtration and easier cleaning. You still want a bioactive substrate and living plants to cycle the solid wastes that don't get carried out the bottom.

I hope this helps, or at least gets the wheels turning some more