How does this work in OggDude's character editor?
As I understand, it needs to be a piece of gear owned by that character, so you can't Tinker with another player's stuff?
Yeah, for OggDudes the Tinkerer talent can only be applied to something that character has in their inventory.
In terms of the game rules, it's my understanding that you can use Tinkerer on an ally's piece of gear, provided your have the means to access that gear on a semi-regular basis to "maintain" the bonus. I think it's notable that Tinkerer lacks the verbiage of Jury-Rigged in that the later only benefits a piece of gear that you are using, where the former doesn't fuss about who's using the tweaked piece of gear.