Picard doesn't write Sort Composer to TSOC, only to TXXX/composersort. I'm working on Picard's end to try to write a plugin or something that will fix it, but in the meantime (and in case that doesn't work out) it would be nice if MB could read TXXX/composersort.
Update: Looks like it's going to be changed in Picard. Hooray! I was about to resort to a custom tag, when I really wanted to stick with TSOC for Sort Composer.