I've recently updated my database of links between animal taxonomic names and literature identifiers, which now has over 280,000 names linked to some form of identifier (127,000 of these being DOIs). You can see the current version here:
http://iphylo.org/~rpage/itaxon/
As an experiment I've added a feature to list the number of names for each journal. Based on this list (limited to journals that I've found an ISSN for) here are some journals I'd like to see digitised by the Biodiversity Heritage Library (BHL). Note that by digitised I mean beyond the 1923 cutoff applied to many journals. This will mean negotiating with the journal publishers, but in a number of cases these are scientific societies or institutions, some associated with BHL. Given that major partners in BHL have made post-1923 content available, it would nice to extend this to other key taxonomic journals.
Revue Suisse de Zoologie
Revue Suisse de Zoologie has published nearly 10,000 taxonomic names but has essentially zero digital presence, which is extraordinary. Another Swiss journal, Entomologica Basiliensia is also an obvious candidate.
Revue de Zoologie et de Botanique Africaines
Revue de Zoologie et de Botanique Africaines has published over 5,000 names, and given the interest in providing information resources for Africa (e.g., http://www.mendeley.com/groups/1681811/bhl-africa/) this seems an obvious journal to scan completely.
Bulletin of the British Museum (Natural History) journals and books
The Natural History Museum [formerly British Museum (Natural History)] is a member of BHL so I'd expect it to have better coverage of it's own publications in BHL. There are gaps in journals such as Bulletin of the British Museum (Natural History) Entomology, which means there is a significant chunk of research published by Museum staff that simply doesn't exist digitally. At one point The Natural History Museum renamed the journals and moved them to Cambridge University Press, resulting in further gaps in digitisation. It's interesting that museums that haven't changed the title of their publications (such as the American Museum of Natural History and the Australian Museum) have better digital coverage than the NHM, which has flirted with various title changes in the last few decades. The Museum also published a series of monographs in the 20th century, many of these aren't in BHL.
Memoirs of the Queensland Museum
The Memoirs of the Queensland Museum is an important journal (> 3,000 names) but has only early issues scanned in BHL and recent issues as PDFs on the Museum web site (vulnerable to link rot when the site gets redesigned, as I've discovered to my cost).
Russian journals
Russian journals contain large numbers of taxonomic descriptions, but their digital presence is patchy. Springer has started to publish translations online (e.g., http://dx.doi.org/10.1134/S0013873810050155 in Entomological Review, which is a translation of an article in Zoologicheskii Zhurnal), but much of the Russian literature seems unavailable in digital form. BHL has spread from it's US-UK origins to BHL-Europe, BHL_China, and BHL_Australia, maybe it's time for BHL-Russia?
Summary
There are huge holes in the availability of taxonomic literature (where I equate "availability" with being digitised and online, free or otherwise). But on the other hand I've been pleasantly surprised by just how much taxonomic literature is online. It looks quite feasible to link at least 300,000 animal names to digital publications.
The journals I've highlighted are just a few obvious candidate for scanning. I suspect that as one goes down the list of taxonomic journals the rate of return will decline, to the point where scanning entire journals will be less efficient than scanning targeted articles.
