And here's the finished thing, in case anyone is interested:http://www.filedropper.com/slowdownarrowfix-mod
Appears to work like a charm! One thing that might look like an oddity is the 2 turn duration, but that's only really noticeable outside of combat, and it's because 1 turn will be "eaten" when the enemy turn comes around. Oil itself lasts for 1 turn, but renews whenever you're in oil, so 2 turns on an enemy = 1 effective turn.
Threw me off for a second (because I did the testing outside of combat).
Obviously, it's a proper non-.pak mod, so it goes into /data/ and can be freely edited, distributed, picked apart, learned from, whatever.