How would I do a QoS setup where a certain low-priority data stream would get up to X Mbps of bandwidth, but only if the current total bandwidth (of all streams/classes) on this interface does not exceed X? At the same time, other data streams / classes must not be limited to X.

The use case is an ISP billing the traffic by calculating the bandwidth average over 5 minute intervals and billing the maximum. I would like to keep the maximum usage to a minimum (i.e. quench the bulk transfer during interface busy times) but get the data through during idle/low traffic times.

Looking at the frequently used classful schedulers CBQ, HTB and HSFC I cannot see a straightforward way to accomplish this.