Modify movement speed through .json on an item?

This is based off of a silly whim I had while swimming through a river to escape zombies.

Question. I was thinking of making a neoprene mermaid tail item, that would slow you to a (literal) crawl on land, but be pretty awesome underwater.

This was inspired by those silicon mermaid tails ala±+mirror.jpg?format=1500w

I was looking through the .json and at the wiki, and this is the closest current thing I found:

“type” : “ARMOR”,
“id” : “swim_fins”,
“name” : “pair of swim fins”,
“name_plural” : “pairs of swim fins”,
“weight” : 680,
“color” : “dark_gray”,
“covers” : [“FEET”],
“symbol” : “[”,
“description” : “A pair of rubber flippers worn on the feet which improve swimming speed while greatly impeding the wearer’s ability to walk.”,
“price” : 2000,
“material” : [“plastic”, “null”],
“volume” : 5,
“warmth” : 5,
“encumbrance” : 15,
“flags” : [“VARSIZE”, “WATER_FRIENDLY”],
“coverage” : 85,
“material_thickness” : 1

But contrary to the description, it doesn’t actually seem to do anything to help one swim - it just has the water_friendly tag, which AFAIK … just modifies morale. Its not even terribly encumbring.

Is there any way, through .json, to actually add movement speed while underwater? I could then add in some godawful feet/leg encumbrance to signify that one is basically immobile on land when wearing one.

Currently no, fins’ effect is hardcoded by ID.