Well, I have never heard of oil being used as any type of chelating agent. I suppose that the nature of the oil swishing around in your mouth would be able to mechanically lift minute amounts of surface mercury off of your amalgams, but it seems pretty benign compared to chewing solids like food or gum, particularly as your mouth is more likely to be open when chewing a solid like either of the latter. To clarify, if your mouth is open, you are more likely to inhale the released mercury vapor.
It's just not something I would worry about. It's not like any of these oils contain free thiol groups that would chemically mobilize mercury, or a dithiol like DMSA that is useful in chelation.
Amalgams are something to worry about overall, though.