Make another extension reject a call in queue (a.k.a. priority call)

Reposting my original question as follows: is it possible to suspend (i.e. to put on hold) a ringing call on a target extension for allowing a priority call to ring?
What I want is not a Message Waiting Indicator feature, for in MWI both calls are signalled to target ext. I want only the priority call to be ringing.
Thanks
Francesco

PS: original post follows.

I am using a queue to route inbound calls at a customer’s system. Ring strategy is “ringall” as customer asked me so. But customer wants also the following: while there is a call ringing to all extensions, he want to be able to make an internal call to one specific extension making it stop ringing from the queue and start ringing for the direct call.
I know it sounds quite non-standard, and I’ve already told him it’d be really tricky to do it; but I’m wondering (for my curiosity) if it’s possible in some way to make another extension STOP ringing and reject a call like it were busy or unavailable.

Francesco