I understand what you are saying, but I just don't consider a trike a motorcycle. If a disabled person rides a trike to still enjoy a motorcycle I'm all for it, but I still probably will not wave at them unless I'm feeling generous that day or I know them to be disabled. I also do not wave at people in cars whether disabled or not.
Probably the only exception to this rule for me would be a trike I saw built once that used a Peterbuilt engine from an 18 wheeler that they turned into a trike with huge truck tires and all. Anyone riding that beast deserves a wave.
Found the video of that trike:
Peterbilt Trike - YouTube