Some phones - like the Motorola X - have an option where if someone rings a second time within a set period, then the call is allowed through. So, it would be possible to do the same sort of thing by monitoring Phone Events. Record the number of incoming calls, if the time period is appropriate, then set the ringer mode
Alternatively, don't give clients your mobile number. I find it only encourages them. I use CallWeaver to ensure the phone only rings for certain people. Clients are told to call my office number. My old Panasonic answering machine had two mailboxes; if callers pressed #1 then it went to the urgent one, and the machine called my mobile after the message was left. You could do similar with an IVR these days, possibly even connecting the call to your mobile after the message "Press 1 to reach my mobile. If it's not actually urgent, I'll charge you $200"