There are a few un-documented helper functions that plugin developers can use. They should appear in the documentation page: http://dev.piwik.org/trac/wiki/Plugins/GlobalFunctions
For example: - how to write SQL in Tracker code (where Piwik_* doesn't work) - expand documentation on the smarty functions - more..
To build some plugins it would be useful to easily access a list of developer friendly flags:
* isAdmin * isSuperUser * isAnonymous * isView * ?
we'll update http://piwik.org/docs/plugins/functions/ at some point