"i reject your reality and substitute my own" - adam savage. i suspect my final words maybe "well shit, that didnt work"
instead of perfection some times we just have to accept practicality
they did produce one 4 the u series though. i saw a flyer somewhere. cant tell you what model winch was used though.
The D4 winch will fit. The later model D4N winch will fit better though. If I had to guess, I would say it was not recommended because the controls interfere a bit with the 6U/7U seat, especially if you have a seat tank. The gear selector on the D4 winch sits a bit higher than it does on the D4N so you have to raise the seat up by about 3". Also, the brake lever doesn't stick out as far as it does on the D4N, not quit clearing the seat.
That being said, there are plenty of 7Us around with D4 winches.