Since using attachTo even on a local object is broadcast to players. I think it would make sense to have either a local param in attachTo or a new function. lightAttachObject is also another function similar attachTo but is local for lights?
Ex: attachToLocal || attachTo [object2, offset, memPoint, followBoneRotation, isGlobal]
I also had a need for such feature and found a hacky workaround. The way to solve it is to attach through a intermediary local object (createVehicleLocal)
[OBJECT 1] <-attachTo-> [LOCAL OBJECT] <-attachTo-> [OBJECT 2]
When one of two objects in attachTo is local, attachTo doesn't broadcast it.
This hack also lets you locally change positions of global network synced objects, just attach it to local object and move the local object, no network messages involved.
Beware though, there is a thing called "error" that accumulates the more object's state differs from state received from the server. I think this error affects precision of interpolation or something, also causes yellow chains to show up? Basically its better not to abuse this too much.