Hauling a motorcycle between rear bumper and trailer

I wouldn't condemn the rear, just yet. Just eyeballing the pic, the 400# bike will take about 250-300# off the front. It's plenty heavy, I'm guessing you're talking about a 7% weight reduction.

I would want to know the tongue weight of the proposed boat and trailer before I made the decision.

If you're intent on putting it up front, no big deal. If you can't buy a factory front receiver for that chassis, you just need to find a local welder/fabricator to build one for you.