David,
Yes, I did see that your edit-mode [det] button would take the selected text and treat it as the name of the tiddler (the one to be displayed within the details "pocket"). This is ideal for the situation where the other tiddler already exists. For my writing and editing process, by contrast, I'm almost always finding that the thing I want to tuck into a details "pocket" is initially right there in the same tiddler.
In fact, prior to seeing your solution, I had been looking forward to trying to reverse-engineer a variant of the excise button that dovetails with telmiger's details widget (rather than transcluding or linking), though I would be terribly slow at the work.
And despite being very fond of telmiger's details plugin with all of its css flexibility, I do see advantages in your model, including the elegance of typing (and reading) <<det "FAQ">>, plus the subtle built-in asterisk-link to go straight to the tiddler in question.
Still, one other feature/option that may be of interest (based on spending lots of time working with <$details> widgets, which allows summary to be specified as a parameter): ability to prefer the caption (or some other specifiable field), as the preferred summary text, defaulting to title only if caption is unavailable.
On your site, this tweaked variant seems to get the macro to show caption (as summary) if it exists, and to default to title if not:
\relink det tid
\define det(tid:"") <details><summary><$transclude tiddler="$tid$" field="caption"><$view tiddler="$tid$" field="title"/></$transclude> <$link to="$tid$">*</$link></summary><span class="indent1"><span class="tidgraybk"><$transclude tiddler="$tid$" field="text" mode="block"/></span></span></details>
-Springer
[Sorry for blinking you all with a post-delete-repost; I found a small typo in the macro, which should be fixed now.]