Dst, people wanted to make an event revolving around buying slaves, leashes were made to support that, but in here you partly focus on the usefulness of the leash powers and dismiss the slaves concept. (the one-way transportation devices can be a separate subject unrelated to the existence of slavery if you want, rather than making them, as-they-are-now, more important than the reason why they were made, if they are that important, then they can have a life of their own, they don't need to have fate bounded to that of slavery)
I agree that the (slave) leash must be earned, but I would go in a bit different way.
For instance, I would like a 'recipe' for that leash of magical nature, similar to what you suggested, but not in requirements-increasing behavior, because I don't understand why would that happen other than some practical results you wish to see achieved on the field - for me it has to feel natural.
- A player should be able to make his own leash (requirements: bla bla bla, 4000 heat, drop of blood, bla bla bla, some special kind of metal), and then he could sell it to anyone in any way he/she wishes. Requirements should be nasty.
- Player who has a slave should be able to have a bigger variety of effects of enforcing and control upon the slave. (random suggestio - pulling all the chat entries made by player in the last X hours, like hearthepoor spell? Thus tracing what your slave was doing, was he an obedient slave, etc.)
There also should be a method to enforce someone to have a leash, but it needs to be very hard to perform. (because what would be the point of having leashes, if you can just 'sell your services'? Aspect of non-consensual must be present for it to make sense)
Because of them being able to be sold individually, someone might ask 'what of the slave auctions'?
Slave auctions would offer that 'free' way of selling your leash to someone like it was so far, but it would also be a wild card because your control of the deal was minimal.
All that leash nature background reminded me of 'damane' from Wheel of time, magic (sort of speak) users enslaved by a certain type of leashes.