SML / SMP / NAPTR question

I'm prototyping some  BDX and other technologies and ran into the following issue:

In SML, one URI  pattern is defined, to be used when a retrieving metadata for a specific  document type:

http://<hash over  recipientID>.<schemeID>.<SML  domain>/<recipientID>/services/<documentType>

Section 2.2 of the  PEPPOL SMP suggests SML/SMP also provide an interface to retrieve a ServiceGroup  for a given Participant Identifier.  I
n this case we don't know the  document type.
What is the URI pattern to use to retrieve a ServiceGroup and where  in SML or in SMP is it defined ?

When using NAPTR  U
records, the second occurrence of
recipientID
in the URI would no  longer needed as the record can directly reference  a recipient-specific resource on the metadata server, it is not just a  domain.  That is an improvement.
But that raises the question  how to combine that referenced resource identifier with a variable  documentType.  Can the regular _expression_ be used?

The
/services/
substring in the URI seems to be unnecessary in SML already.