FLAC isn't a supported codec, sorry.
At this moment, it's not. My interest is in creating a channel that will do so.
When I got my first Roku with USB support, the only way to play media through the USB port was to go to thenowhereman site and install the private USB channel he made (and still makes) available. Soon after, Roku included a USB channel in the channel store. Some sources have stated that Roku contracted with or hired thenowhereman to adapt his USB port channel for use as the official Roku one.
I note that in the case of video files, thenowhereman's USB channel supports only MK4 but the Roku channel also supports MOV and WMV. So clearly various codecs can be supported by Roku if a channel is set up to do so.
I've got the developer's kit. I figure that I should be able to produce a channel for playing FLAC files via the USB port.
The point of my original posting was to see if anyone else has successfully created such a channel. If so, I don't need to reinvent the wheel. If not, perhaps someone has done some preliminary analysis and investigation which might be useful to me in doing the job myself.
(I really thought that was fairly clear in the posting.)