I don’t want to understate the problem, but it seems to me the problem still comes back to QoS. The user should be able to choose the QoS he can accept for voice traffic. If that level is constantly being monitored by either a front end black box or by the router itself, it could then generate a busy signal to the PBX/voice control device whenever the QoS is exceeded. This would then cause any new traffic to overflow to the PSTN.
In this fashion, one would never need to calculate how much bandwidth is available. Of course, until the router vendors step up to the plate, or some black box is developed to do this, we still need a way to calculate it, even if it’s just the amount of overflow.