JD,
First, the 884 uses only one flavor of HexFET, the IRL3103.
http://www.irf.com/product-info/data...ta/irl3103.pdf
There is no simple way to interface to a higher current device without modification to the 884. You would need to decode directional data as well as PWM switching. At this point you should know that the FET used in the 884 is capable of 64 amps of drain current, each. With three devices in parallel you have the potential to control 180 amp loads. The downside of this implementation is heat sink, of course. The power dissipated in the devices causes a heat rise within the case of each FET. Without an adequate way to rid themselves of the heat, they eventually would self destruct. That is why the 884 is rated at 40 amps with fan.
The design criteria for this controller is first to read the incoming PWM and determine from that decode process the magnitude and direction information. Then provide drive and directional commands for the output FETs. The rest of the circuitry provides the needed power supply conditioning, tally LED drive, PWM input conditioning etc. If you could provide a suitable heatsink would the 180 amps be sufficient? Remember that all the tabs of the FETs are not at the same potential. If you are looking for even higher currents, there is always the possibility of removing the FETs and using the gate drive to control external FETs. At this point I should tell you that handling high currents requires a good working knowledge of electrical design. Even the 40 amp design of the 884 required considerable knowledge of the tradeoffs and device design specifications of the FETs chosen as well as the layout of the controller. High currents can cause conductors to vaporize so there is some danger invovled for amateur experimenters. Proceed with caution.