Kinda brute force, but I tested it last night with FLAC files and enabled debugging of format conversion and yes it works. You'd have to create other custom-convert.conf rules for any other type of lossless file that you may have and disable all but Mp3 streaming for them as well. For any lossy encoded formats, including Mp3, I'd leave the default settings. There's no point in transcoding anything lossy to 320 kbps CBR Mp3, as their native formats will already be at a lower bitrate.
Well said. Everything is m4a, flac or doesn't need lame.
If I could presume on your help as to what the m4a code would include?