Not sure how that mod does it, but the changes to holster/sheath action have made it sorta possible. Key discovery is that the action’s “merge sheath, scabbard, and holster functions” code means you can define your own flag and it won’t care that you made it up on the spot.
{
"type" : "ARMOR",
"id" : "backpack_alt",
"name" : "tactical backpack",
"weight" : 633,
"color" : "green",
"covers" : ["TORSO"],
"to_hit" : 0,
"storage" : 20,
"symbol" : "[",
"description" : "A small backpack, good storage for a little encumbrance. Use it to stow away assorted junk.",
"price" : 3900,
"material" : ["cotton"],
"volume" : 8,
"cutting" : 0,
"warmth" : 5,
"phase" : "solid",
"encumbrance" : 10,
"bashing" : -4,
"coverage" : 30,
"flags" : ["BELTED"],
"material_thickness" : 2,
"use_action" : [
{
"type": "holster",
"holster_prompt": "Put item away",
"holster_msg": "You put away your %s",
"multi": 2,
"min_volume": 1,
"max_volume": 10,
"draw_speed": 80,
"flags": ["SHEATH_MISC"]
}
]
},
There’s one caveat though. It doesn’t seem like defining multiple instances of the holster use_action works. This means you can’t have multiple separate compartments with different max capacities.
Even if you could resolve that issue, the volume fuckery only lets you store X items between Y and Z volume. No way to adapt it to true DF-style (store any number of items up to a set volume capacity) backpacks yet.