Jim,
I think you could accomplish your mod. What you would need to do would be to buy two liquids jugs. Modify one of them by inserting a bottom drain. From the bottom drain you'd connect the line. You'd have to drill out the bottom side of the main unit and make a slit about 2 times the height of the line. Then you'd need to rout out a bit of the floor to accommodate the line as it feeds back under the head to drain into the black tank. The Oatey plug that you'd use to cap the tank will require modification to accommodate the line. You won't have a perfect gravity fed slope, but it should be all right.
To change to vessel collection, pop off the quick connect, and lift out the modified jug. Put in the standard vessel and you'll be in business.
Seems like a lot of work for not much payoff. But that's how I'd do it.