Made more changes to the TOC-friendly titles (<title type="trunc">) on CC's instructions, and in the process noted a presentation problem: sorting was being done on the original title, not the TOC-friendly title, although the latter was being displayed, so the ordering was wrong. Fixing this isn't just a matter of switching the sort parameter because some items don't have a TOC-friendly title at all, so this is my current solution:

<xsl:sort select="concat(upper-case(./descendant::tei:title[@type='trunc']), upper-case(./descendant::tei:title[1]))" collation="http://saxon.sf.net/collation?class=TitleSortComparator"/>

which seems to work fine.