Emulating a scroll wheel
I created a file named /etc/X11/Xsession.d/98custom-devterm, containing the following line:
xinput --set-prop 9 'libinput Scroll Method Enabled' 0 0 1
So now, when I hold down the middle mouse button the trackball acts as a scroll wheel. This means I can’t use the middle mouse button of course, but I don’t think I will need it, and having a scroll wheel is a lot more useful.
ETA: yes, this does work for both horizontal and vertical scrolling (I just checked). Of course, with the DevTerm’s aspect ratio you wlll be doing a lot more vertical scrolling…