I havent used a teensy, as yet !
On my Nano+ENC28J60, I have about 500bytes left, running either a 2 universe sACN or ArtNet, and also the ability to boot it into a config mode that serves a webpage to a browser for set up of IP etc
So, it is possible, and stable
Once I get this up and running nicely I shall post some code, I might even try and do something like you are doing to see if it is possible, I reckon you could get 2 universes out to DMX (but that would need 2 uarts), so one is definately possible