Following on from Jon's follow-up, Hubert Figuiere has released Exempi 1.99.0, now based on Adobe's XMP SDK.

His blog post includes a nice snippet of code showing how to apply a CC license to a PDF:

`

include <exempi/xmp.h>

...
XmpFilePtr f;
f = xmp_files_open_new("test.pdf", XMP_OPEN_FORUPDATE);
XmpPtr xmp = xmp_files_get_new_xmp(f);
xmp_set_property(xmp, NS_XAP_RIGHTS, "Copyright", "(c) ACME Inc., some rights reserved"
" - This work is licensed to the public under the Creative Commons Attribution-ShareAlike "
"license http://creativecommons.org/licenses/by-sa/2.0/");
xmp_files_put_xmp(f, xmp);
xmp_free(xmp);
xmp_files_close(f, XMP_CLOSE_SAFEUPDATE);
`

Excellent news for the community, and for the continuing saga of XMP.