There appears to be an error when assigning global user rights to a global account with spaces. Earlier I tested it by assigning my test account global IP block exempt:
(show/hide) 16:08, 1 January 2012 PeterSymonds (talk | contribs | block) changed global group membership for User:Peter Symonds from global-ipblock-exempt to (none) (done)
(show/hide) 16:07, 1 January 2012 PeterSymonds (talk | contribs | block) changed global group membership for User:Peter Symonds from (none) to global-ipblock-exempt (testing bug in global rights interface; will revert in a few seconds)
The following message was thrown back:
"The global user PeterSymonds_(test) is not active on the wiki you specified ( aawiki ). You may be attempting to assign rights to the wrong user!"
Despite this message, as the log shows, the right was assigned successfully. Furthermore, I didn't attempt to assign the right on aawiki; it was assigned on Meta-Wiki.