This cetainly does the trick and you are free to merge, but I do have a few comments/querries.
It has taken me a while to wrap my head around why your original instinct of simply "tweaking of the types in the HscMapper.paf file" didn't work. I think part of my confusion lies in the naming and actual behaviour of the exposureFromImage function in dat_butlerUtils' cameraMapper.py. The doctstring desciption is: "Generate an exposure from a DecoratedImage or similar", but it seems that "or similar" effectively translates to "or Exposure" (any thus not really useful since that would imply you've already got an Exposure!) as these are the only two with the getMetadata attribute that gets called on the "image". This function is called within _standardizeExposure if the mapper's python field contains the string "Exposure" AND the persistable field contains the string "Image". Since both MaskedImage and Image contain Image, these types could be passed into the exposureFromImage function which will then fail on the call to image.getMetadata(). It's difficult to say for sure what the specific intended use was meant to be as the @param[in,out] item (lsst.afw.image.Exposure) docstring in _standardizeExposure implies the input should already be an Exposure. I wonder if this exposureFromImage function should behave more like your standardizeCalib function (i.e. to accommodate the Image and MaskedImage types as well)? Otherwise, might we run into the case that such "custom" standardization code will be duplicated in other similar cases (e.g other obs_XXX packages)? Sorry for the sidetrack...I realize this is out of the scope of this issue, but for the lesser-initiated this is clearly a potential source of confusion.
My other comment (actually related to your contribution!) is that I find your if/else clause a little bit misleading. You provide a raise RuntimeError("Unrecognised python type: %s" % mapping.python), but this would not catch any non-LSST defined image types that still contain the string "Image" (e.g. "bogusImage" would not be caught). However, I think any non-recognized type would be caught by lsst::daf::persistence::Formatter before this code is ever executed, so this point may be moot regardless.