I believe easiest way to think of this problem is to remember that firewall rules must be present that specifically enable fasttrack between designated pairs of interfaces. (Interface lists can muddle the underlying behavior, so pretend for the moment that they are not there to use.)
Ensure your configuration is designed such that the customers you wish to be queued are connected via one or more dedicated interfaces, which interfaces carry no other category of traffic that you would wish to have fasttracked, nor can they be bridged with other interfaces that do. (Even if you must share the same physical interface, e.g., a wireless AP, partition the traffic using a virtual AP.)
Then set up your fasttrack rules to include all interfaces that carry fasttrack traffic, but include none of the interfaces that carry queued customer traffic.
I am personally running this configuration on wireless towers with through-traffic to other towers, and it works perfectly.
Once you have it working, you can define interface lists to achieve the same effect with fewer firewall rules, if you think that will result in added efficiency; but beware of using any of the pre-existing lists that have dynamic membership, as it may easily produce inconsistent results.