Loosely inspired by FSF SacVentral and Zade's Backpack on Chest, this "mini mod" has a simple goal - to put your backpack on your belly, freeing the backpack slot for something more immediate, like a parachute.

Features
These are some of the features I missed in all other implementations and the reason why I wrote yet another such mod from scratch: preserving ammo counts in magazines during chest/back switch, no magazine-refill cheating preserving attachments/magazines of weapons in backpack during chest/back switch full Multiplayer compatibility, incl. Join-in-Progress (spawning new unit or taking control over existing one), Respawn, even TeamSwitch should work works with Zeus-controlled AI units To put a backpack on the chest, ACE-self-interact, Equipment, "Backpack on Chest". To do it the other way, "Chestpack on Back". If you have both a backpack and a chestpack, you can swap them using "Swap Chest/Back packs". While carrying a chestpack, the soldier is forced to walk (balancing reasons).

Limitations
The ammo/weapon preservation is possible due to the use of get/setUnitLoadout, as I found no way to store this metadata into an actual backpack (attached to chest or on the ground). The backpack you see on the chest is just a visual model, not a backpack you can access. This is also why I can't create ACE interaction on backpack objects to place them on the chest directly or an interaction to drop a chestpack on the ground.
If you want to pick up / drop / access a chestpack, you need to do it manually from the backpack slot.

To access the chestpack of a dead or unconscious (ACE medical) unit, you need to ACE-interact with it and move the pack onto the unit's back, then you can access it like a regular backpack on the ground / via ACE unconscious inventory.

I also wanted to add the chestpack actions when swimming, but I was unable to figure out why exceptions[]={"isNotSwimming"} doesn't work on/inside ACE_Equipment - if you have a clue, I'm all ears.

Vanilla Arma 3 compatibility
There's none, but the actual logic uses vanilla functionality available even to just missions, so feel free to rip it out and create Action menu actions via addAction for your mod/mission. Just see the code, it's very simple.

Download
chestpack_v1_0.zip (2.3KB)
This mod obviously depends on ACE_interact_menu from ACE3, so it's good to have it.

Released under CC BY 4.0,
https://creativecommons.org/licenses/by/4.0/

Enjoy!
(And sorry for the slight visual glitches, I adjusted the chestpack position as best as I could, visually.)