[cleszkie]

I was just about to suggest cutting out the mounting location on the pillar, fishing the lost plate out through the new hole in the pillar with a magnetic probe, re-affixing the plate cage to the backside of the cut out mounting location, and then welding the cut out back into place in the pillar followed by some light grinding and bodywork. But now I see you don't want to damage fresh paint. You may not have another alternative. you really need to have full access to the plate and cage to do the job right. The limits of the paint damage can be minimized such that a small amount of touch up painting should be all you need.