Simple solution:

<recordSchema> in the request is a simple string.

It can contain either the short name of a record schema (as mapped in
Explain) or the full identifier.

We could change the semantics to:

It can contain one or more short names or identifiers.  If more than one
is supplied and the first is a 'wrapper' schema, then the second and
subsequent should be returned within that wrapper.  If the first is not
a know wrapper schema then the server should treat it as a choice of any
of the schemas.  [currently an extension]


<recordSchema>mets dc rec</recordSchema>

If the server doesn't support this, it will return a diagnostic saying:
I don't support record schema "mets dc rec".

If the server does support it, but doesn't support one of the individual
schemas, it could return:  I don't support record schema "rec"

If it's a choice and the server doesn't support any of the schemas, it
can return multiple diagnostics, one for each.