With the Network Edition (said you had NE 4.5.7) there's
Outlook & iSync connectors.
I haven't had trouble myself, but some people are reporting slight issues deleting events in thunderbird/lightning setups:
/forums/administrators/11622-thunderbird-lightning.html
Connectors aside,
It would also be simple/easy from your end to train them in the excellent web-client. (Or is the web-client not an option for you? too much load etc?)
Then when
ZDesktop goes GA, training concepts haven't changed.
You've then moved from supporting 6 or so clients to just one interface, plus your admins can see what's currently set/easily change settings for the users when they call in.
Let them know if they want their own thick-client tell them they're on their own to support it...ya I know, saying that works better in theory than actual practice, because you'll probably still have to give out how-to docs & you can't just ignore someone asking for help...