Garage floor mounted trailer hitch?

[Roadrunner]

Believe me I've tried stopping right on the tipping point, but it's steeper than it looks in the car and there's a very fine line between not tilting and coming crashing down.

I think the safest solution is what StilIIC mentioned earlier. I remember a Dingo digger I hired some time ago, its trailer had a hydraulic damper on it which made the tilt very smooth and calm - very much like this:
http://mgaguru.com/mgtech/trailer/th404.htm

Wonder where I could get one of those from or fitted? (which I'm guessing is a bit more heavy duty than boot hatch gas struts )

[greenMachine]

How about clamping some timber at the tipping point as a temporary wheel stop? Difficult to do when loading, but by gradually moving the car back when loaded it should be easier to identify the optimum location for the wheel stop (use a jack or wheel stand(s) while experimenting so the trailer tip is limited).

This could be complicated if the CoG changes when you get out, but if that is the case make the 'stop' high enough to indicate where to stop, but low enough to drive over when the trailer bed comes down. In this case, the 'stop' could be permanently fixed in place.

Good luck.

[StillIC]

.....Wonder where I could get one of those from or fitted? (which I'm guessing is a bit more heavy duty than boot hatch gas struts )

I'd be looking to get a gas strut used as a car or small truck shock absorber, sized correctly for length and stroke, with an eye at each end. I wouldn't even worry too much about the rate, initially, as it is impossible to be 'too soft' compared to nothing, will not make it any worse, and I don't imagine it could be 'too stiff', given the angle. If too soft, just add more dampers of the same design until the rate is about right.