We currently have "informal" identifiers that are the latter portion of the PURL (lilly/hohenberger/Hoh010.000.0049 or archives/cushman/P03412). We will retain this scheme until we find something significantly better.
We may eventually assign a DOI for each item, but we don't have a need to do that yet.
Some older collections don't include a fully-qualified ID in their OAI records. For example, in Hohenberger, we have Hoh007.013.0016 instead of lilly/hohenberger/Hoh007.013.0016. We will have to support the old forms of the ID.
NOID avoids the use of vowels, to prevent the unintentional creation of words (which may be misleading, as in a Variations ID like bad6666). It also avoids the use of lowercase 'l', to minimize confusion with the number 1.