Alexei or Rob, any thoughts on whether this would be worth doing?

It doesn't seem unreasonable. We already have a test that does something like this - AboutFlagsHistogramTest.CheckHistograms - which checks that all the entries in about_flags are included in the XML enums.

One thing worth noting is that there are histograms that are only documented internally - but they're a tiny minority of them - and maybe don't use the tester infrastructure.