This sounds like a more general version of what I want for
one-photo-per-page galleries, where each page has previous|up|next links
(like this plugin) and the index page has a list or grid of thumbnails
(\[[!inline]] with a specially modified template perhaps). I'll watch this
with interest! --[[smcv]]

This is a nice idea, I do have my gripes about the imeplementation.

Assuming that the index's list is in mdwn format is not ideal. I guess the
other way to do it would be to make the index be a directive, something
like: \[[!trail pages="foo bar baz"]]. Assuming that a flat trail structure
is enough, otherwise you'd have to get more fancy.

The trailinclude seems a bit redundant with inline, and wanting to inline
together all pages in a trail for printing or whatever seems like an
unusual use case anyway?

The !trail directive could be simplified to just \[[!trail my_indexpage]].
But I wonder if needing to add this directive to every page is the best
approach. Alternate approach would be to make the trail index cause 
breadcrums to be automatically inserted at the top of every page on the
trail. (You'd have to use a directive to define the index for that to work.)

--[[Joey]]