Follow up thoughts... (from someone with no clue about ARB functional requirements)
I can't imagine it requiring a whole lot of volume to engage one...some pressure, yes. Maybe add a small tank (purged propane torch cyl?) to the hand pump.
If you are crafty, wire the switch to activate a LED/buzzer that will alarm when line pressure has dropped to whatever min spec is for the diff. (time to stroke, or kill the switch and do what ya gotta in open-diff mode)
Of course for all the cheapness, this does not one bit of help if ya gotta do tire service or other volumiferous chores.