I think, usually each hotbar slot should have its unique common event (whether it's called directly or via item/skill doesn't matter here), so I don't see the benefit yet.

However, I quickly made a plugin that puts the value of item/skill/equip/common event of the latest hotbar interaction into a Variable. Hope this helps!

Change the Variable ID at the top to your needs, and then save it as a plugin with any name.

/*:
 * @target MZ
 */
(function() {
const variableId = 1; // <--
const _GameParty_onHotbarItemEquipWeapon = Game_Party.prototype.onHotbarItemEquipWeapon;
Game_Party.prototype.onHotbarItemEquipWeapon = function(weaponId) {
    $gameVariables.setValue(variableId, weaponId);
    _GameParty_onHotbarItemEquipWeapon.call(this, weaponId);
}
const _GameParty_onHotbarItemEquipArmor = Game_Party.prototype.onHotbarItemEquipArmor;
Game_Party.prototype.onHotbarItemEquipArmor = function(armorId) {
    $gameVariables.setValue(variableId, armorId);
    _GameParty_onHotbarItemEquipArmor.call(this, armorId);
}
const _GameParty_onHotbarItemRegularItem = Game_Party.prototype.onHotbarItemRegularItem;
Game_Party.prototype.onHotbarItemRegularItem = function(itemId) {
    $gameVariables.setValue(variableId, itemId);
    _GameParty_onHotbarItemRegularItem.call(this, itemId);
}
const _GameParty_onHotbarItemCastSkill = Game_Party.prototype.onHotbarItemCastSkill;
Game_Party.prototype.onHotbarItemCastSkill = function(skillId) {
    $gameVariables.setValue(variableId, skillId);
    _GameParty_onHotbarItemCastSkill.call(this, skillId);
}
const _GameParty_onHotbarItemCommonEvent = Game_Party.prototype.onHotbarItemCommonEvent;
Game_Party.prototype.onHotbarItemCommonEvent = function(commonEventId) {
    $gameVariables.setValue(variableId, commonEventId);
    _GameParty_onHotbarItemCommonEvent.call(this, commonEventId);
}
})()

Yes, actually I had the same idea, and I'm already working on it :)

Either by plugin command or script call, these are explained here: https://aerosys.blog/action-hotbar