One thing that got a lot of discussion on the skeet shooting web sites a couple of years ago regarded the use of Green Dot 20 gauge loads in the Briley subgauge tubes. While the loads used were perfectly good and safe in a standard 20 gauge gun, tube sets were cracking at a faster than normal rate. It was pretty much agreed upon (I think even by Briley) that it is better to use a slightly slower powder, like Unique or Universal Clays in the subgauge tubes, rather than Green Dot.
Green Dot is still ok, so long as you're using Federal hulls, or loading 3/4oz 20ga. Alliant has plenty of recipes for either of those, and the pressures are all pretty low. Green Dot in AA or STS hulls for tubes is a no-no.
According to the Kolar there is a recommended limit. Here's what they say in the information included with the tube sets they sell, "We strongly recommend 20 gauge chamber pressure be maintained at less than 10,700 PSI." Kolar also provides a list of recommended 20 gauge loads based upon using Remington RXP & STS hulls and Winchester CF & HS AA hulls. The recommended powders included in the list are, Universal, SR7625 & WSF. The list includes recipes for loads that range from 9,800 PSI to 10,700 PSI.