Steve,
I have used "reinforced" elbows in my carrer. I can't say that I have with B31.1 but definitly with B31.3. Some companies that I have worked for in the past thought that adding a re-pad to a trunnion attached to a elbow was a great way to solve local stress problems due to the loads on the trunnion.
Personally I feel the prefered method of reinforcement is to just specify a heavier wall elbow fitting to provide the reinforcement you need instead of having the pipe fabricator try to fit an eliptical re-pad to the heal of the elbow. When using heavier fittings remember to taper bore as required where welding to thinner compoments.
When solving local stress problems, my first choice is to increase the trunnion size as much as possible and if that's not enough, then I use a heavier schedule elbow.
Adding a trunnion and/or re-pad will increase the stiffness of your elbow thus decreasing the SIF values. There are no recognized procedures to calculate these adjusted values. If your company doesn't have a procedure in place for dealing with this situation then use FEA analysis or use conservative SIF values based on an unreinforced elbow. If you use a heavy wall elbow, base your calc's on that heavier wall.
Good Luck,
NozzleTwister
Houston, Texas