Not so sure i want to keep trying that, triggering a scan multiple times might cause lockout....it has in the past.
The URL to just get favs for .m3u is this:
http://x:8089/devices/TVE-Comcast_SSO/channels.m3u?filter=favorites
So...replace ?all=true with ?filter=favorites maybe? in the curl command
that is assuming you can pass arguments to their scan function