suggested flag and description:

"xmp - Enable support for Extensible Metadata Platform (Adobe XMP)"

current usage:

local use flags (searching: xmp)
************************************************** **********
[- ] xmp (app-misc/tracker):
Adds support for XMP metadata extraction

[- ] xmp (gnome-base/nautilus):
Adds support for XMP metadata

[- ] xmp (kde-misc/tellico):
Adds support for PDF/XMP metadata using media-libs/exempi

[- ] xmp (media-gfx/eog):
Adds support for XMP metadata

[- ] xmp (media-gfx/exiv2):
Adds support for Adobe XMP

[- ] xmp (media-gfx/geeqie):
Adds support for XMP metadata

current ways of providing the support:

media-libs/exempi *nix port of Adobe's SDK and/or media-gfx/exiv2 using
the "xmp" USE flag using expat