"Can I reasonably compare tons per square inch proof-marks (or in this case, CIP shell ratings) to PSI tensile-strength measurements in a way that allows me to judge what a "reasonable degree" of margin would be for safe use in an older gun?"
NO. Too many variables Lloyd.
And this has nothing to do with whether the recoil of the load is appropriate for 100 year old wood.
What we know about pre-WWI guns and loads:
1. The gun makers said the guns were safe with the shells available in that period
2. Smokeless powder shells were in no way "low pressure".
20g 7/8 oz. 2 1/2 Dr. Eq. BULK Smokeless pressure was 8000-9000 psi; 7/8 oz. 2 1/2 Dr. Eq. DENSE Smokeless was about 11,000 psi.
3. We have a pretty good idea as to the wall thicknesses the makers chose.
4. The makers had a "safety margin". In the Birmingham Proof House Trial there was NO dimensional change in the 19th - 21st barrels, all Pattern Welded, until more than twice the Definitive Proof (DP) charge. As above, in the Second Phase trial, the 12th barrel (because of ties) Foreign Pointille’ Twist failed at 5.77 times DP and the 13th (last) Foreign Four Rod Crolle’ failed at 5.74 times DP.
5. My study, and one unpublished, showed NO evidence of the "because of low cycle fatigue, barrels get weaker over time" myth.
6. Fluid steel barrels DO have a bit less than twice the tensile strength of pattern welded.
7. Bad things happen to barrels over 100 years through misuse, inappropriate loads (steel), lack of cleaning, getting dropped, etc.
8. There is NO definitive NDT for pattern welded barrels (but I think we are getting much better at evaluating their integrity).
What do I think? How much of a margin of safety am I comfortable with assuming I will be shooting next to a grandchild?
