ahhhhhhhh, this has promise!
coming off the matrixctrl that feeds back into /monome/grid/led/set $1 $2 $3 :

this routes only LED “on” messages (if $i3 == 1...) down to an eventual bank of column checks (the example is column 0, with if $i1 == 0), which will each feed into their own zl.reg ! print is there as a reference for me. zlclear will clear the zl.reg buffer if the key is unpressed.
I’m choosing this approach because it seems that zl.reg only remembers one thing at a time, so each LED has to have its own. which means this has to be monophonic for now.
next step is to time the bangs just right – one with the half-brightness column draw, then one immediately following the column clear. hopefully this will retain visual continuity.
I’ll report back!