Turns out Google Books does support searching by ISSN, using ordinary fielded search syntax, although I don’t believe it’s documented anywhere.
Mostly what you’ll find is digitized bound journals from libraries (that is, digitization of some volumes of the journal, probably not all of them, which may or may not have full text access). Sometimes things that physically look like monographs but are published serially also get ISSNs, you might get some of those too, not sure. Has to be in GBS, and GBS has to have ISSN metadata for the record, not sure how often that happens.
http://www.google.com/search?hl=en&tbo=1&tbm=bks&q=issn%3A0008543X
with or without hyphen, both seem to work.
http://www.google.com/search?hl=en&tbo=1&tbm=bks&q=issn%3A0098-7484
http://www.google.com/search?hl=en&tbo=1&tbm=bks&q=issn%3A00987484
But don’t try to put phrase quotes around the value, that (oddly?) doesn’t work, with or without hyphen.
http://www.google.com/search?hl=en&tbo=1&tbm=bks&q=issn%3A”00987484″
Bibliographic Wilderness 
Hopefully this ISSN searching capability doesn’t come as a surprise to people; it’s one of the labelled fields in http://books.google.com/advanced_book_search and that form generates URLs accordingly. I guess you could go through all of the options on that search form and document the CGI params…