I believe the issue revolves around the intended function of the device rather than its actual construction. If the intent is to use the circuit breaker as an OCPD then the height restriction would not apply.
If the intent is to use the circuit breaker as a manual means of disconnecting conductors/eqipment from their source of supply then the breaker is acting as a switch and the height restrictions apply.
Definitions from the 1999 NEC:
Switches.
General-Use Switch. A switch intended for use in general distribution and branch circuits. It is rated in amperes, and it is capable of interrupting its rated current at its rated voltage. ((This is what the SWD rating of a breaker is for))
Isolating Switch. A switch intended for isolating an electric circuit from the source of power. It has no interrupting rating, and it is intended to be operated only after the circuit has been opened by some other means. ((this would apply to a service disconnect device))
Motor-Circuit Switch. A switch rated in horsepower that is capable of interrupting the maximum operating overload current of a motor of the same horsepower rating as the switch at the rated voltage.
Disconnecting Means. A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.
Service Equipment. The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s) and their accessories, connected to the load end of service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.
