LC has prepared a proposal for support of holdings information within a MODS

The basic approach is to incorporate holdings information into the MODS
<location> element.

There have been recent proposals to use a MODS <extension> element for
holdings information. This is not an interoperable approach; however, there
really hasn't been any alternative, since the top-level extension element
has been the only available hook to extensibility.

The new proposal is intended to provide some basic holdings elements for
interoperability as well as extensibility of holdings information for more
detailed information.We think that these earlier proposals can be retrofit
to the proposed hook within <location>.

We welcome comments and discussion on our proposed approach.

--Ray Denenberg