it wouldn't HAVE an ECCN, but an ECCN would be used to describe the software. most software implementing any form of encryption algorithms up to a certain threshold would be 5D992 or 5D002 (i've seen matlab as the former, firefox as the latter). anything 'uncategorized' can get binned as EAR99, but that will likely flag review if you export hardware containing something with that code.
the short answer is most likely a formal identification of Octave's ECCN has not been obtained. So, anyone needing a classification to use Octave could follow the instructions here:
http://www.bis.doc.gov/licensing/do_i_needaneccn.html
Specifically, section 3.3 partway down the page "Get an official classification from the Bureau of Industry and Security (BIS).
" would get you an official answer. otherwise you could go through the 'self-determination' part in 3.1
once that's done, Ian, I'm sure the group would appreciate you sharing the results to save anyone the effort in the future.