I don't care about the technological reason for the rising base ISO. I only care about the practical implication for me and it's that I have to juggle lens hoods, camera bags, ND cases and ND's.
There is a partial solution as I said before: permanently attach an ND filter to each lens
and use a permanent exposure compensation setting
. That will give you less light detected by the sensor, exactly as if (as you seem to want) the sensor were simply less efficient at detecting light in the first place, since lower quantum efficiency is the only thing that gave older sensors a lower base ISO speed.
Actually there is another option with many cameras; Olympus ones in particular: it seems that many cameras with a minimum "ISO dial" setting of 200 actually have a lower base ISO speed (measured by highlight handling), a bit over 100. So their raw files when exposed at EI 200 have about an extra stop of highlight headroom (to the horror of some forum participants.) With such cameras, you can usually safely expose at about EI 100 even if set to the minimum of 200: the in-camera JPEG's will come out too light, but the raw files will be fine with the right processing.