As I understand the reasons for gantries is to make the distance longer between the trailing edge of the CB and front edge of the rudder. The rudder is therefore working in less disturbed wash from the CB, this construction detail/design has become very popular amongst fast, small skiffs which have an extremely small wetted hull area in the first instance, so steering is a premium consideration. A 16ft Cat well trimmed is practically self steering and IMO doesn't plane (overtake and surf it's own bow wave) So there shouldn't be a steering issue like that of a monohull. However, the rudder/steering could sometimes be affected by the wash of the CB but we do have TWO rudders so it would be very rare for them both to either cavitate or just loose performance.
There is also possibly the argument that the increased measurement between the CB's and rudders would also increase the natural turning circle of the boat and as every Cat sailor knows, tacking isn't one of our fastest qualities so this might be another reason why gantries haven't been used.
I haven't looked at the rules this morning but to my mind if the gantry was permanently fixed to the hull then it would form part of the hull, but if it were bolted and removable then I would think the gantry could be classed as an appendage. In this case very large gudgeons or pintles. However, John is correct the best bet is to correspond with the TEC team if this matter is going to be a major concern.