The solution is to use the totaldiscs metadata. If it exists and is greater than one, you know it's a multi-disc set.
I want something similar to this, but not quite this. I would like to use <totaldiscs> >= 2 as the condition to include in the filename "(Disc <discnumber>)". So that single-disc albums don't have this qualifier added.
Squeezebox uses the modtime to work out which is your 'newest music'. It also helps me mentally locate related music in the continuum of my memory!
Hence, I strongly agree: bliss needs to have an option like flac's --preserve-modtime.
When I reordered my old CD collection alphabetically instead of in order of acquisition (when I got married - :-) I lost this important mental connection. I miss it. I got it back with Squeezebox.
And actually I'm sometimes tempted not to use Bliss (good though it otherwise is) because of this problem. For some reason, every now and then it chooses to replace artwork and old stuff pops into my New Music section. Very undesirable.