After a lot of searching, I finally found a reliable way to stream from PC to Roku. I currently run TVersity Media Server on a PC streaming through Xbox, but trying to find a way to get all TV content (Netflix, Hulu, local downloaded media) coming through a single source (Roku).
While a little clunky, PlayOn
looks like the answer. It's clunky because (1) you need to install VLC media player and have it running and (2) it looks like media libraries need to be manually updated (but this is beta functionality, so that may change soon). But I've installed it and it works flawlessly, with no media conversion - it transcodes on the fly. This is perfect for me - almost everything I'm streaming is AVI, and no way I am going to convert it all.