Not on a J/30, but for a similar situation (5ft draft, 5'8" water needed) we used a 30ft, 2" tow strap and modified a hand cart as a trailer nose wheel. If you can line up the trailer for a straight shot, there's no need to mess with 50' of steel... hand carts are cheap and easy to come buy, can be rated for big loads, and allow quick attach/removal.

Using a winch would be another option, better management of the speed, particularly at 50ft.

This is all assuming your ramp is steep enough for the trailer to roll down when empty. Obviously the weight of the boat will cause it to roll down, but for retrieval, you may be right in needing something to push with.