My mistake. I’ll answer my own question for posterity.
There exist the methods
TEfficiency::GetEfficiency
https://root.cern.ch/doc/master/TEfficiency_8cxx_source.html#l02462
and
TEfficiency::GetEfficiencyErrorLow and TEfficiency::GetEfficiencyErrorUp
https://root.cern.ch/doc/master/TEfficiency_8cxx_source.html#l02513
One can then get these values and use TH1::SetBinContent and TH1::SetBinError
https://root.cern.ch/doc/master/TH1_8cxx_source.html#l08350
taking the larger of the two efficiency errors as the TH1 error, since TH1’s can only handle symmetric error. A TEfficiency cannot be used for this ratio (Data/MC) plot, since the number of passed events may be greater than the total events (not binomial in nature). And two TGAE’s cannot be extracted and divided since there is no method to do so (?)
Regards,
Andrew