It does seem to make sense to allow the `_baseclass` to override the indexing. I did check scipy with your changes and didn't see anything the looked related to this. Hmm... could you add a note here and we'll give this a try.

Looks like the code in `MaskedArray` could use a cleanup, if only for clarity.

Curious that `_data` and `data` are the same thing, likewise `_mask` and `mask`. I wonder why that was done? Backward compatibility? Intent to deprecate one?