In the latest pre-release we've added the ability to have a single Custom Source that can contain both XMLTV mappings as well as Gracenote mappings, removing the need to create two Custom Sources.
Existing sources will be defaulted to XMLTV-only to ensure guide data is not disrupted, but all new Custom Sources will automatically notice Gracenote mappings in the m3u and use them.
This setting can be adjusted at any time in the Custom Source settings:
Awesome, thanks! Definitely helps with source mgmt.
Its been a long time coming. This should mean that I don't have to have a separate custom channel playlist if I need to use xml file data now
Nice! 
Of course it'll make some new work for stuff like PLM and FastChannels, but will be totally worth it! Looking forward to giving this a whirl when I have the opportunity.
Good news! In development, I've been able to get this to work and have had excellent results. Details can be seen here.
Tomorrow I'm going to promote the changes to PLM prerelease and then test in my production Channels environment for an extended stretch.
Following your lead here.. I haven't tried it yet- but look forward to implementing in FC.. would make management quite easier!
