On version: 2.0.0-alpha+git.869de4e
Maybe it’s already possible, by editing settings.json and adding some magic stuff on the appropriate section? There’s always hope for this kind of thing.
I tried to change the order of the toolbar buttons that way, but it didn’t work, though, so, probably not.
I kind of feel that the whole screen is not homogeneously naturally “reachable” with the stylus (not a MyPaint-specific thing, but a general painting-program thing, at least on common tablets, rather than Cintiq types). So some buttons, particularly blend modes and undo/redo would be better more around the middle of the toolbar, or even aligned to the right (assuming they’re not alone there, but with the other buttons I have currently there filling some space to the right as well).
Somewhat related to that, to me, that button on the lower-left corner to change the brush ends up being completely useless, it just seems awkwardly “too far.” Maybe having it around the middle would make it more naturally reachable, but to be honest, the odds are that I’d just keep using the brush group docks.