Thank you Matt for your precise answer
It's a pity MC doesn't support IPTC tagging.
While EXIF tags are usually considered "read-only" (containing the "original" facts of a picture) - even if some do have a "write" purpose - IPTC tags are on the other hand used by professional photographers to tag their images with their "additional" information and somehow remain "industry"-compliant (especially with agencys' databases).
Since most the softwares that come with digital cameras are real crap (even those coming along with several thousands dollars "high-end" pros' cameras), MC could come as a genuine replacement for those... but EXIF/IPTC tags (re-)writing is a requirement.
Maybe something "to keep behind the ear"
BTW, have you ever thought of allowing users to make mixed "audio/photo" programmable playlist (something like: at time "t1", play "track" from time "start"; at time "t2", display "image" using blending "effect"; at time "t3"...) ? You have
everything you need in MC (out of the "timed" playlist) to make of it the best
musicalized/commented slide-shower I'd ever seen (including configurable IR remote control, sound DSP-ing, etc...).