See for example http://meta.wikimedia.org/wiki/Special:CentralAuth/Synthebot ... this bot has an SUL on Meta.
If you go to http://meta.wikimedia.org/wiki/Special:GlobalUserRights/Synthebot to adjust the rights (to make it a bot) the second dropdown defaults to metawiki. This is a wiki where Synthebot has an ID. But there is an error message displayed:
The global user Synthebot is not active on the wiki you specified ( ). You may be attempting to assign rights to the wrong user!
This error is somewhat misleading. (note the empty parens)... if you press Edit User Groups, it then presents you the rights (if that user has an account on Meta) to edit.
Suggest that this message be suppressed or that the code check to see if the user is present on (metawiki) rather than on ( ) .... knowing that the user really doesn't have an id on Meta would be useful.