So with an ISOless camera you should attempt to ETTR at base ISO.
Jack
Now
I'm getting a bit confused, Jack.
Surely with all cameras, whether they are ISO invariant or not, one should attempt to apply ETTR at base ISO for best results, if the required shutter speed and aperture allow it.
Likewise, with all cameras, whether they are ISO invariant or not, one needs to apply the ETTR process
regardless of the ISO setting chosen, in order to get the best results, the best results being the lowest noise in shadows and mid-tones, without blowing wanted highlights.
For example, if I'm using a D7000 or D7200, which I would say are pretty close to being ISO invariant cameras, and I decide to use ISO 800, not only because I need a fast shutter speed, or because the light is not good, but also because I want to clearly see what I've shot on the camera's LCD screen, and/or share the images with others in the field, then I still need to apply the ETTR process at ISO 800 in order to achieve the lowest noise and the best review image.
In other words, any exposure which does not push the histogram to the right, whatever the chosen ISO, will result in more image noise.
Where's the confusion? The fact that one might not
need to use a high ISO with an ISOless camera, if one doesn't feel the need to be able to clearly view the shots taken on the camera's LCD screen, does not mean that the ETTR principle does not apply if one
does choose to raise ISO, rather than underexpose at base ISO.
However, that is not to deny that the ISOless camera has clear advantages. But that is a separate issue.