We are using empty ALT, because if you use alt="<space>", IE will show ugly empty hints on images hover. If "alt" is empty, then this doesn't happen.
P.S. Added JIRA RFC to review the problem: https://jira.jboss.org/browse/RF-8690
What is involved in editing and re-compiling source if I need to do this anyway? I have asked the client to reconsider this option, but I know what way I would decide if it's a choice between IE and ugly hovers vs screen readers reading out URLs.
Recompiling is easy: http://community.jboss.org/wiki/HowToBuildRichFacesSnapshotsManually33x . For changing you'll have to replace in renderers all empty entries of "alt" attribute with spaced one.