I've not had an issue with all capital letter users either, except on the console. There I would fail a login using any random lower-case characters and then you can login with the one you want.
I ran AIX, Solaris (2.5 to v8), HP-UX 11.11, Dynix 4 and RHEL 5 & 6 quite happily like this. Our application required all capitals because it was originally grown on an IBM mainframe running VM/XA & MVS/ESA (gosh I'm old!
) and they assumed every user account was only capitals.
Login over the network seemed never to be a problem. Are we worried about users defined in LDAP / AD perhaps etc.? We'd gone away from user login to the server by that point so they logged in to the application instead.
Robin