I fixed my own problem, by putting in an "Execute Shell" command:
mv ios/build/Release-iphoneuniversal/MySpecialProject.framework .
Then in the post-build action, "Archive the artifacts", I was able to just archive "MySpecialProject.framework/**"
This seems to work quite well. Since each time Jenkins runs the job, it creates a new workspace, I don't have to worry about issues with "mv". Works fine.