You would have to take an object that isn’t the player character, specifically link that object to to the pc to give a bonus similar to an aura or a buff, and then be able to remove that link on the fly. Possibly they could piggyback the mechanic off the fact that the thrall is on follow, if the code allows. Programming can be finicky like that. Essentially, it’s more moving parts so to speak, so more things that can go wrong in a game that already has plenty of bugs and glitches.
On the other hand, a thrall is a mobile object in the game, and object that is specifically intended to carry things for the player, it has attributes. Inventory space would likely be one such attribute, that, while it’s still possible for things to go wrong, I would assume, should be as simple as changing a value.
That’s simplifying it a little, as it gets trickier when spawning various levels of quality and all that, but in a base sense… It would make more sense to alter the thrall itself rather than attempt to create new buff specifically for bearer thralls that doesn’t really even make sense in terms of realism or practical function.