I use the following and it works really well with a little setup and effort. As well, my recorded shows are under "Shows" on Boxee Box.
Windows Media Center as PVR (shows are recorded as .wtv)
MCEBuddy 1.1 is always running in background. It performs a commercial skip and compresses the large .wtv file to a tidy .mp4
At this point BB can play the files, but they are not named correctly to end up under "Shows". For this I use TV Rename.
Now for the part that pertains to you:
TV Rename requires adding your TV Shows (so it knows what you want to track). I have it set to scan a couple times a day. TV Rename recognizes the files and puts them in the correct show folder. Subsequently the file now has all the correct info for BB to recognize it.
It has an option to "Look for airdate in filenames" with this I have been able to get it to recognize my recordings pretty flawlessly. Here is an example of my tv show filenaming:
American Idol - Winner Announced - 2011-05-25-19-00-00.mp4
As you can see it includes Showname, episdoe name, and airdate with time (time is not needed but thats how WMC does it).
TV Rename uses theTVDB for its data (which I believe Boxee Box also uses for tv shows). As a nice added feature TV Rename will also download nfo files (if you want). They are working really well.
How are your filenames coming out of MythTV?
Hope this helps
TV: Samsung LN46A550P3F (HDMI), Firmware: latest "pushed" version, Router: Linksys WRT54G2, Connection: Wired, Media Source: Win7 PC