The new look Biodiversity Heritage Library includes articles extracted from BioStor, which is a step forwards in making the "legacy" biodiversity literature more accessible. But we still have some way to go. In particular the articles lack the obvious decoration of a modern article, the DOI. Consequently these articles still live in a twilight zone where they are cited in the literature but not linked to. DOIs are becoming more common for taxonomic articles. Zookeys has them, and now Zootaxa has adopted them (and will be applying them retrospectively to thousands of already published articles). Major archives of back issues digitised by Taylor and Francis, and Wiley, for example, also have DOIs.
One obstacle to assigning CrossRef DOIs to articles in BHL is the convention that DOIs are typically managed by the publisher of the journal. But in a number of cases the publisher may no longer exist, the journal may no longer be published, or the publisher may lack the commercial resources to support DOIs. In these cases perhaps BHL could adopt the role of publisher?
Another approach is that adopted by a number of other digital archives, whereby the archive assigns DOIs to articles, but these DOIs are registered not through CrossRef but with another DOI registration agency, such as DataCite. For example the Swiss Electronic Academic Library Service (SEALS) archive assigns DOIs to individual articles, such as http://dx.doi.org/10.5169/seals-88913.
There are some limitations to not using CrossRef DOIs, in particular, you don't get the full benefits of their metadata-based services such as getting metadata from a DOI, discovering DOIs from metadata, or citation linking. But all is to lost. Some services support both CrossRef and DataCite DOIs, such as http://crosscite.org/citeproc. For example, for the DOI 10.5169/seals-88913 we get some basic formatting:
Perret, Jean-Luc. (1961). Etudes herpétologiques africaines III. Société Neuchâteloise des Sciences Naturelles. doi:10.5169/seals-88913
This still leaves us lacking some services, such as finding DOIs for articles cited in a manuscript. However this is a service we can provide, and will have to anyway if we want to find all the digitised literature available (e.g., archives such as SEALS as well as numerous instances of DSpace). My preference would be for CrossRef DOIs, but if that proves problematics we can still get much of the functionality we need using other DOI providers.