Here is the secret to how to get grease into the hinges.  

With the steel tube welded in place, a void surrounds the hinge pin between the two bushings.  All you have to do is drill a hole through the hinge pin so the grease can get to the center of the pin.  Note the small hole in the side of the hinge pin.  That is where the grease gets into the void.  Applying additional pressure on the grease with the grease gun's pump action forces the grease out into the bushings.  There is no other way to grease a hinge bushing and actually get the grease inside the bushing where it will do some good.

Oil impregnated bushings

Custom machined hinge pin

Threaded hole for zerk

View of 1/2" ID tubing and hinge body drilled to 1/2"

These instructions below show how to rebuild hinges using solid 3/8″ diameter rod without the zerks. When rebuilding without zerks, both ends of the rod get welded. With zerks, only the end opposite the zerk gets welded.