PMH has an operation listMetadataFormats which lists the local names and
any other information it has about the formats it supports.
(view source to see the response without the helpful XSLT)
Eg, they have the problem, but don't even bother with a registry.
On Wed, 2009-05-20 at 15:30 +0100, Ray Denenberg, Library of Congress
> From: "Dr R. Sanderson" <[log in to unmask]>
> > There isn't one, but the protocol has the same requirements as SRU and
> > OpenURL.
> I remain confused. If the protocol has the requirement to specify a schema
> identifier, there must be some way for an implementor to (1) find out the
> appropriate identifier for a particular schema, including finding out that
> there isn't one; and (2) register an identifier for a schema when there
> isn't one.