I saw some functional handcuffs a while back, but I can't find them again, so now I'm asking here in the hopes that there's some sort of option that lets you turn on the arms-pulled-behind-back thing for a sim

Were you thinking of the "Have Sim Arrested Mod" at MTS?

https://modthesims.info/d/662800/have-s ... d-mod.html

No - because that just lets you arrest sims - I need to have someone handcuffed and still able to sit down and walk around and do other things for a story I'm doing

I'm not sure if such a thing is possible, at least from an animation standpoint.
What you saw may have been a posepack, for a different game in the series, or... well, the most likely place I imagine such a mod or piece of CC would be hosted (if it does exist) would be on the more NSFW modding sites.

There are also a few ways to glitch sims into the EA handcuffs, I'm not sure if they can be directed to do anything once they're stuck in that pose though.