The OnOpen trigger is "...not to be used in the QlikView Server environment..." (Qlikview Reference Manual section 30.3)
It seems to work if you use the IE Plugin client. I guess if you use the IE Plugin, you are effectively opening a local copy of the document as you would with desktop qlikview. It doesn't seem to fire with the Java or Ajax client.
On the other hand:
"The following triggers will work as usual in the QlikView Server environment regardless of which client is used:
If one of these events takes place before you need to control your selection, you could trigger your macro from there.
I'm building a welcome page which will have messages and disclaimers, and a big 'Continue' button that executres a macro determining which sheets a user can see.
If anyone knows a better solution, please let us know!