This should work without any problems in Tableau. I don't see anything wrong in your R code.
summary(yourModelHere)$adj.r.squared will return a single value and Tableau will replicate it for however many rows you have in your view.
Is the problem 1) you are getting an error 2) you are not getting a value at all 3) you are getting N values but you only want to display it once ?
If you could clarify, it would help in finding the solution.
Has anyone managed to solve this problem? I'm curious about the same thing.