Looking at the picture of the commercial tongue weight device, could you not make something using a cylinder with oil in the lower part and install a 1500 psi hydraulic gage in the hyd fitting on the lower half, and be able to sit the tongue on the shaft of the cylinder and be able to read the tongue weight. Chock the wheels first.
Yep I did just that. I put a gauge on my HF portopower clone so I could tell how much force I was applying to things. Then I found one of my smaller rams was right close to 1 square inch ram area. Voila! Instant scale. The one square inch makes it easy to read weight directly on the pressure gauge. I put the ram under any trailer coupler and see what it takes to lift, that is the actual weight measured on the gauge. Any size ram will work but then you have to do the math if it isn't 1 sq inch to convert.
I used it to great advantage figuring out how to position my motorcycle, atv and Bx24 all in an enclosed trailer. Turns out, motorcycle and atv go side by side in the front then the tractor is backed in. Due to weight of the backhoe pulling it in forward resulted in too little tongue weight.