Originally Posted by dddd

You can visibly see the difference in cog spacing between 7s Accushift and 7s Shimano, and that's enough for me to rule it out.

Back when I raced with 7s Accushift Command Shifters, I wanted to use Shimano freehub parts and their lighter aftermarket clones like ti cassette cogs and lighter freehubs.
I absolutely had to re-space the Shimano-compatible cassettes in every case, lest the cable adjustment need a different setting for each end of the cassette.
I sanded down plastic and aluminum cassette spacers as needed, on the basis of repeated road testing.

And as for using the 6s shifters with a 7s cog stack, what will keep the shifter from slipping down from the largest to the second-largest cog when there is no detent? Is the rider supposed to hold their thumb on it or change to frictioin mode whenever a steep hill threatens...