This question often pops up on the boards and has to do with the way global notes are implemented.A global note is implemented as a notifier and as such the server for the notifier exists in a seperate process so resource files that are loaded in the client side of the notifier will not get transferred to the notifier server process and so only avkon resources (in avkon.rsc/avkon.rsg) are guaranteed to be supported in the server process* and hence the only CBA buttons that can be used are those in avkon.

Of course it is entirely possible to add a new cba structure to the data that is marshalled between the client structure and the server but this is not currently implemented and probably will never be due to issues around localization and the fact the global notes should strongly be discouraged as they block  access to the device until the note is dismissed.

(*this is sort of correct, its implemented slightly differently underneath)