I bet something skyhook like could be made from one of these HF cranes, and they are fairly cheap.
Pickup bed crane
Smaller pickup bed crane
I got bored last January and did as you suggested above. Looked at jib cranes, davit cranes, engine hoists etc. Saw the pickup cranes at HF and waited for a big coupon sale and got one. Already had the winch laying in a corner. Set the hydraulic jack, boom and boat winch aside. Welded up a new boom, mounted the winch, fastened it to the welding bench and I was set.
Swings 360 deg and reaches the bench, mill, shaper, slotter and everything in between.
Weak point appears to be the thin wall vertical column. I'll bet I could pick a half ton off the end pulley, but don't believe that tube would like it where the knee brace comes in... Also- it has no slewing bearing- just metal on grease on metal, but swings fine for me with or without a load.
Starting from scratch one could design and fab as wanted, but this got me going, done and works well.
And I do wish I had Dr. Clyde's bridge crane...