I use the following command to enable modifiers for the drive toolbar (accordingly to the default modifiers when clicking on folders). Unfortunately clicking a drive button multiple times with changing modifiers doesn't work as expected (sometimes a modifier seems to be pinned).
@keydown:none
Go DRIVEBUTTONS=fixed
@keydown:alt
Go DRIVEBUTTONS=fixed NEWTAB
@keydown:ctrl
Go DRIVEBUTTONS=fixed OPENINDEST
@keydown:ctrlalt
Go DRIVEBUTTONS=fixed OPENINDEST NEWTAB
@keydown:shift
Go DRIVEBUTTONS=fixed NEW