I think you will need to download the master playlist to tmp:/ and parse it, this shouldn't be too difficult to implement within the Roku channel. then point the video playback function at the parsed file or, you could do the parsing/rewrite on your server
Unless this changed very recently, Roku's HLS implementation won't work with local playlists. It has to be hosted. If it hasn't changed, at the risk of derailing the topic slightly, consider it an enhancement request to support M3U8s via tmp.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)