Well, after some testing and hair pulling, that's not it. That "call load" variable uses a formula that measures some kind of load, but not simply the number of calls in the queue. So there is no way to use it like I need.
I also tried combining "agents available", "agents idle", "calls waiting", with and, or, etc., to no avail, there are too many combinations possible.
Now I'm thinking of using an IP trunk loopback restricted to 10 calls, it seems a bit complicated to program, though.