I hereby nominate the eBay item linked to in post #13 (Lucky 13 at that) as the bad idea of the year.
As far as the original question goes, I can't image adding another 300 lbs to the receiver, and then adding on the tongue weight
24" further back. Talk about a recipe for a disaster.
Now, if Billy-Bob wanted to pull his 12' bass boat and put some bait boxes in the bins, it MIGHT work, but not with anything we have here, at least that I am aware of.
Maybe that's why it's on eBay, it's not the first bad idea I have seen there.