FMP7 Server and clients

I would like a button, which when pressed pops up a value list. The
chosen value will then be supplied as a parameter to a script.

This will be in the document header, not body.

The bit I am struggling with is how to make the pop up list. I know how
to make a field pop up, but the problem here is what field should I use?
I can't just create a field to supply this parameter, because the
value must be private to this user.

Oh hang on, global fields CAN be private sometimes can't they? Dim
memories... I'll go and play, but I thought I may as well post this
anyway as it is an interesting question and I'd be grateful if there any
other thoughts on how to implement this behaviour.

Thanks

Luke