Carrying a dirt bike on the hitch of bumer pull TT?

I agree with the above. Lowering the ball will not change the tongue weight to any big extent.

The whole idea, in my opinion, is a very bad idea. The addition of all that weight, far behind the trailer axles on that small of a trailer, will make the tongue so light as to probably make the trailer un-towable. (is that a word? :W) I would live with taking the topper off and hope it doesn't rain.
Barney