I know that there is not a key, but is there a mod that adds an Arvel Plantation Slave key?

I find a few things:

Nevena's Twin Lamps & Slave Hunters 1.5.esp
Juniper's Twin Lamps (1.1 Tribunal).esp
FreeMoreSlaves.esp

These all contain a MISC object with a name that contains "Arvel" and "key". But I'll leave it up to you to decide if they match what you want.

Thank you