Since no one has offered an answer to this yet, I think the reason is maybe it simply can't be done. When we met with the Honda folks in the focus group they held in Denver this spring, one of them was the chief engineer for the RL. One of the attendees asked about putting on larger aftermarket wheels and why Honda didn't go with a larger wheel on the RL. The engineer responded with a number of reasons/answers to these questions... here's a summary of what I remember (he was very clear about these issues and was very adamant about why he - and Honda - strongly oppose putting aftermarket wheels on the RL):
According to the engineer, it took several tries by the wheel manufacturer to develop a wheel that would satisfy all Honda's requirements - which included safety, durability, appearance, and economics. In the engineer's words, "The wheels on the RL are the largest you can safely put on this truck." The engineer pointed out that in the real world, many people will overload a truck at one time or another. Honda tested the RL for safety and durability with the bed overloaded up to 50% beyond its rating under various conditions (not just a flat out road, but under other stressful conditions like towing, slalom and such). Since this weight is ultimately carried by the wheels, the RL's wheels needed to be able to carry this load without failing. The wheels that come with the RL are able to do this and as the engineer stated, "Aftermarket wheel manufacturers do not have to adhere to the same federal safety regulations, testing and so forth auto manufacturers do, so there's no way to tell whether an aftermarket wheel won't fail under stress, so you're taking a chance whether they will or not and Honda won't warranty a failure caused by an aftermarket product."
Without getting into the engineering details, as a wheel diameter increases, the strength and/or amount of the material also needs to increase in order to maintain the wheel's ability to handle stress. A good example of this is in vintage car racing. I have a '68 XKE with wire wheels, but I don't race it as some XKE owners do. For those that do race their E's, the smart ones have a set of solid steel wheels they mount on them for racing since the wire wheels won't hold up under this type of continued stress (not just higher speeds, but the lateral stresses incurred during hard cornering and torsional stresses during hard braking and acceleration). The wheels on the RL were designed to deal with an overloaded, stressful driving situation without failing. Most aftermarket wheels are unlikely to have this kind of safety built into them.
The RL's wheels are also designed to collapse a certain way during a side impact which is all part of the crash safety mechanism that earned the RL a 5-star rating.
The 4-wheel drive, stability control and possibly the anti-lock braking mechanisms (don't quite remember for sure on the brakes, but I think so) all utilize wheel rotational speed and forward motion speed to determine when and how these systems react to driving conditions. The systems are calibrated for the wheels and tires the RL was designed with. So, by changing the final wheel/tire diameter and therefore the rotational speed of the wheels, these mechanisms are getting erroneous data which will most likely cause the system to over or under react to changing driving conditions depending on the situation. This could mean the difference between avoiding an accident and making it worse.