Recently a new RA (registration Agency) became operational under the DOI umbrella: BSI Identify (https://identify.bsigroup.com/about/). They offer a very simple service: a BSI UPIN (Universal Persistent Identification Number), which is a DOI for construction materials. It will survive product discontinuations so it will allow you to get what is known about a product such as the water pipes in your house long after the product is fabricated, installed, operated or even decommissioned. A manufacturer can provide product data to BSI Identify to create UPIN identifiers for their products and attach it to them as a barcode for example.
I think that for a Digital Specimen identifier a little more would be needed in terms of services. There may be a need for a reverse lookup for a physical specimen identifier for example. What would be the minimum services needed to be provided by a RA for digital specimen identifiers?