Or the way to detect an action:
if (player performingAction "rearm") then {...}; or EH performingAction to avoid loop (limited but sensible list, not a keyDown/up check, already existing).

OR, install ace 3 mod (for now). They disabled it. ;)

I don't like ACE3. This addon is far too much possessive. It disables too much native Arma commands, like "put" "take" and so on.

You can disable it, its modular mod

ACE3 disables the pickup action, because it horrendously breaks with mine detector-type items:
http://feedback.arma3.com/view.php?id=20997

Items in ACE have to be of this type to show up in the Virtual Arsenal, because mine detectors are the only item type that have a neglible side effect

There is no way to add items that do nothing that is hard coded to the game:
http://feedback.arma3.com/view.php?id=23925

On topic:

This Rearm eventhandler seems as pointless as the Take and Put eventhandlers are.

Something like LoadOutChanged is what's really needed in my opinion.

https://resources.bisimulations.com/wiki/Category:VBS:_Event_Handlers#LoadOutChanged_.28Unit.29

"This Rearm eventhandler seems as pointless as the Take and Put eventhandlers are."

Well, LoadOutChanged returns only 1 param, and that is:

"Object - Object that had its equipment changed"

Basically player object. Calling Put and Take pointless when in fact they provide the item put or taken as well as ability to override and information about containers involved in transfer... is a bit of a nonsense, don't you think? Or maybe I misunderstood and you gladly go through the pain of storing and comparing all player's inventory before and after "LoadOutChanged" event just to find out what is that being put or taken? In this case my apology.

Sure, LoadOutChanged as it exists in VBS could be improved, but I don't see how another eventhandler that still doesn't cover inventory commands -and therefore the VA- really helps the situation.