You said..."We're using a constant substrate temp of 28 C and we are keeping the substrate wet enough so it will clump but dry enough that you can't squeeze any water from it"...you have the right idea for the substrate. I incubate mine around 74F and have done this for years. The temperatures fluctuate several degrees from daytime to night time because I don't use a proper incubator. I use a human heating pad (the kind that have 3 settings) and put a wooden frame over it with screen on the frame. They hatch at about 8 months. I have 100% hatch rate of fertile veiled eggs using this and about 95% survival rate of the hatchlings at 3 months of age.

You said..."What should the relative humidity be in the incubation chamber?"...never measured it...I can tell you that I use shoebox sized plastic containers. I punch 2 VERY tiny holes in the lid. I fill the containers about half full of coarse vermiculite prepared the way you said. I lay the eggs in rows in dents that I make with my thumb. There will be condensation on the lid and sides of the container....never been a problem. If you take the lid off too often and things start to dry out you will have to add a little water around the edges of the container (not on/near the eggs).

Good luck!