How often do you expect to move a 500lb item? In a production shop making large things I could see that but building something for that kind of a load when your realistic need is maybe 75-150lbs (larger tooling). Then there is the how often would you move it all the way across the shop?
It seems like smaller more focused methods are more practical, which is why you see solutions like those sky hooks. Setting up a much shorter span / lighter duty track between a specific machine and its tool storage would be far less expensive and more practical. Also wheels are awesome, put your heavy tools on a wheeled cart, then move them to your lifting device situated by the machine.
Moving a machine is a rare thing once you get it into the shop. It seems like for most of us hobby guys a gantry crane spanning a shop is one of those cool things to have that you use when you first move everything in and after that just one more thing to hit your head on.
Also I think you are grossly exaggerating the issues with an engine hoist (other than the %#@*!!! legs that don't fit under or around anything). I've moved several pieces of machinery in the 200-600lb range with mine, and while there is a bit of truth to the issues highlighted, it really only comes into play if you are being careless. The momentum issue you mention exists on a gantry as well, except it becomes more wrecking ball like if you let it get out of control.
Yes Engine hoists are awkward, but they do the job, and the space issue is much reduced if you disassemble them rather than just fold them up when not being used. Takes me less than 10 minutes to take mine apart or put it together, and apart it can easily be tucked under a bench, between a bench and wall or in an unused corner.
If I had the shop of my dreams, I'd move the heavy stuff with a scaled down remote controlled version of this.
For what you describe I'd think a small mobile crane would be much more practical, something you move to the work, then do the lifting. Distance movement being done low to the ground on wheels. I love my low to the ground wheeled carts. Cheap, effective back savers.
I think Erik Manie put a skyhook on his rolling tool cart, and it seems to be working for his 16" lathe tooling.