I was thinking about trebuchet configurations. The trebuchet frame could be centered over the turret ring, which would provide both support for the trebuchet mine sling and a convenient area to load a new mine. It may be easier to have the mines launched to the rear, since it would reduce the travel for the "counterweight" which would be provided by a motor or directly from the main tank engine.
Alternatively, the trebuchet engine could be packed inside a cut down turret and the framework attached to the turret, allowing some azimuth pointing.