This is tricky because the views don't keep track of the list of items
that they are displaying. There is a way to get the *set* of those
items, but not the *list*, as the ordering of those items is dependent
on each view, and though they get sorted, their order is only used for
rendering and then discarded.
You might have to override the default implementation a bit to do what
you want. The list view and thumbnail view are both backed by this code
http://api.simile-widgets.org/exhibit/2.2.0/scripts/ui/views/ordered-view-frame.js
Look for
Exhibit.OrderedViewFrame.prototype._internalReconstruct
inside it, find the function called createItem. You can override
_internalReconstruct so that createItem caches the item it renders into
an array, from where your Thickbox can used to find the next item.
David