Based on the description in this document:http://www.iptc.org/std/Iptc4xmpCore/1.0/documentation/Iptc4xmpCore_1.0-doc-CpanelsUserGuide_13.pdf
located on the IPTC website, You may be able to use the Instruction field and annotate it along with any other applicable information, perhaps...
Model Release: Yes/No
From the document referenced above:
The Instructions field is a simple text field that can be used to include any of a number of
instructions from the provider or creator to the receiver of the photograph. Any of the following
could be included: embargoes (News Magazines OUT) and other restrictions not covered by the
“Rights Usage Terms” field; information regarding the original means of capture (scanning notes,
colourspace info) or other specific text information that the user may need for accurate
reproduction; additional permissions or credits required when publishing.
Note: This field is “shared” with the “Instructions” field in the Origin panel of the Adobe
Photoshop File Info field.
I may be wrong in my interpretation of this, but others may be able to shed light on this.