Are there any good local multiplayer games that are downloadable on 360? I was bummed to find that all of the well praised games in that vein from this year (samurai gunn, nidhogg, towerfall) are all being ported to new consoles.

I could move my PC into the living room but I don't have that many wired 360 controllers.

Try looking through the Xbox 360 platform forum (to which I just moved this topic to) for some ideas. Here are a bunch of topics:

https://www.google.com/search?q=local+multiplayer+xbox+360+site%3Agiantbomb.com&oq=local+multiplayer+xbox+360+site%3Agiantbomb.com&aqs=chrome..69i57.6941j1j7&sourceid=chrome&es_sm=91&ie=UTF-8