For some Shimano models, you can get the levers separately. For the M401, the lever doesn’t seem to be available, however:
A non-servo wave lever that is available would the one from the XTR 9100: part number Y1XJ98010.
Edit: this is the GRX lever for comparison:
Ooof, have you seen the price?
It’s carbon though 
But yeah, not really something you would buy just to see if it is compatible with the GRX lever.
Ok, I’ve taken a few photos. I remove only 1 part and cut and rebuild only that part! Photo #2 shows the cheap squared part I did cut to make the lever. The only problem was ot find one measuring exactly 15mm as the removed lever
Right now I’m mostly wondering what the difference between it and a non-servo wave lever with 180/203mm disk is (when using an adaptor to correct the bar-lever distance). I might get one before the year’s out and do my bit of freewheel testing. At the very latest I’ll buy one when I move to the Qu-Ax freewheel hub…
