I thought about doing something like that with my DR walk behind string trimmer. I was thinking on putting it on an arm attached to my loader bucket so I could change both height and tilt with the loader. Also be out front instead of behind the tractor.
Looks like what you did works pretty well. Beats dragging it up and down the bank by hand.
DR makes a tractor attachment 3pt string trimmer, but it is very pricey.