Moss is a great medium for it, also in nature it's not uncommon for plant seeds ending up in moss and germinates.
This happened in my garden, seeds from a flowering Cyperus alternifolius next to it blowing into the moss and germinate.
Cyperus also might be an option for you, its a pretty undemanding plant sp. If you keep a Cyperus from the garden centre outdoor during the summer, it will flower and carry seeds. These you can collect and germinate. It's a grass, if you cut it in time it will be similar to maintaining a lawn. It will rejuvinate and spread and stay small. Note the plant will not survive frost.