Extension for HoldingsHelper: subfield code "r" to include the sequence number
now you can concatenate the fields localid and sequencenumber to build the necessary number for requests against the aleph REST interface
at - least: give it a try...

note: this is only more in the testing state - I guess there is a lot of space for extensions. Only as small example: concatenate the subfields already in the helper functionality so it hasn't to be done o the view script level