holding the reload button to activate a synth seems like a better solution to me. instead of adding another button to push, just use the same one you already use.

Holding square already does many things, could affect existing code or be too hard to implement.

the x/square are definitely busy buttons, but their default usage is "reload" and using a synth reloads your weapon so it seems pretty intuitive to do it that way. All the other uses are context sensitive and are accompanied by on screen prompts telling you what will happen if you hold the button. This is where it could get brilliant: make the "no xxx ammo" message that flashes on the screen when your equipped weapon runs out of ammo context sensitive. Using heavy as an example the out of ammo message cound read as follows: Out of heavy ammo and no synth available: "No Heavy Ammo." Out of heavy ammo and synth available: "No Heavy Ammo, Hold x/square for Ammo Synthesis." with the usual circle animation that draws around the button as you hold it giving you a chance to change your mind. Out of heavy ammo and synth in cooldown: "No Heavy Ammo, Ammo Synthesis Available in xx:xx." giving you a countdown timer to let you know when you can use your next synth pack.

I like this idea but I think using square will cause issues. And so I would suggest maybe holding down circle, doesn't have a hold down function and u should be able to set a consumable to the "hold down circle" hotkey. The idea of your ammo being empty then getting the option worries me some, maybe it's cause I like to use a pack when I have 1 rocket left to save time or I want a fresh pack even tho I have 3 heavy bullets left. I think a combination of our ideas using circle could be very effective