While trying to get a list of indef blocked IPs through ipb_user = 0 betacommands list ( http://tools.wmflabs.org/betacommand-dev/reports/enwiki_indef_ips.txt ) returned a number of weird non-IP entries.
Krenair was kind enough to compile a list of the 57 accounts involved.
It also seems like this issue is perhaps known since November 2008: https://en.wikipedia.org/w/index.php?title=Special:Log&offset=20081116011504&limit=33&type=block&user=MZMcBride
I wonder if these accounts can be registered or what would happen if they were to make edits.
Another question is how many similar accounts do other wikis have?