I had an idea for a spiked chain-wielding Barbarian character, but instead of the usual warrior brute I thought I'd try something different; he was some kind of escaped slave or something that had been treated like an animal his whole life, and the spiked chain he fought with was actually attached to a collar around his neck. He still needed to use his hands to attack with it, but as a bit of flavor text the chain hung from his neck by default and couldn't be removed. Are there rules for having attached weapons like that?
Certainly, there’s no reason why you couldn’t describe a spiked chain this way. Disarming is a kind of niche tactic; it might simply never come up, at which point there’s no real need to mechanically “back up” the description.
But ultimately disarming could happen and as a DM, I’d rather not feel like I “shouldn’t” disarm you even though mechanically you can be. Unless, of course, I felt like giving you that bonus as a freebie for having a cool character concept and because disarm isn’t really that great a move, which I might because both are true.
At any rate, if nothing else, refluffing a Locked Gauntlet provides a huge bonus to prevent disarming. As the spiked chain is a two-handed weapon (+4) and you have another +10 on the bonus, you probably won’t ever get disarmed, and if you somehow do it could be ruled a freak accident where the latch pops open or something. The only problem at that point is explaining why your hand isn’t available (since the locked gauntlet prevents the hand from being used for anything else), but that could be as easy as a shackle on the wrist or just waiving that limitation as not really significant.
On the other hand, you could simply port the Weapon Cord from Pathfinder
It's an interesting bit of flavor. I like the weapon cord idea, though I'd add one small detail as a balance factor: dropping the weapon doesn't prevent you from being counter-tripped on a failed trip attempt (because it's still wrapped around your neck). I'd have no problem allowing this, with that compromise.