but then I need to call the loop like a bajillion times.
Note that my two-liner above produces all histograms in one event loop (by delaying the calls to GetValue until after all Histo1D and Histo2D calls have been made).
If RooUnfoldResponse does not inherit from TH1, you can work around the bug that requires it by defining a little helper object that wraps a RooUnfoldResponse and does inherit from TH1, like in this post.
Hope this helps!
Enrico