Is there a way to configure mod_auth_openidc to use a OIDCProviderMetadataURL that returns in xml format?
No, the OIDCProviderMetadataURL works against a well-defined standardized JSON document that describes the OpenID Connect Provider, see: http://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata
There's no standard that says how to express this in XML, every Provider would do it in its own way so there's no point in trying to implement this in a generic Relying Party like mod_auth_openidc.
I'd advise you to rather use something on the server side that converts your custom XML into a standard JSON Discovery document and work from there.