So far for the mossy tree stump I'm thinking keep the syngonium, add:
- cryptocoryne blassii
- pennywort
- eleocharis vivipara
- ficus microcarpa.
Looks promising! I've had cryptocoryne wendtii do fine I guess in my paludarium (surviving but barely growing), so C. blaassii should be doable. If you can find it, Lagenandra might be the better option though, and it has recently turned up on some suppliers lists here so could be worth another look. The others should all do well as long as you're a bit patient with the ferns.
- For the moss/plant wall I'm thinking:
- Tradescantia
- More creeping fig
- Microsorum that I can transfer from the tank (Inspired by hydrophyte and this post from @foxfish)
- Bolbitis (can transfer this from the main tank)