A few disconnected observations on these issues:
I'm not sure whether it is a good idea, but <facsimile> now has <front>
and <back> children. There seems to be some attempt at providing
structure there? I always wondered how these were meant to be used, as
<front> and <back> can't have <surface> children?
We have thought about using @rend to encode the functions of multiple
graphics ('thumbnail', 'fullsize', 'archive', etc), but it seems abuse.
Filenames help, as do mimetypes, but TEI should help us be explicit. To
me, there is an obvious need for a @type or @function attribute here.
I sometimes felt the need for an @isReverseSideOf attribute on <surface>
to encode a relation between surfaces.