I feel your pain.
I have had success in the past retrieving AWOL metal pieces using magnets.
I might try placing a strong tubular rare earth magnet inside a flexible vinyl tube and slide it into the B pillar cavity through the top screw hole.
If you can somehow pull the plate upwards so you can grab it through the lower bolt hole, perhaps with a dental pick, I think you could then manage to proceed to remount the striker plate.
Also, you could drill a larger retrieval hole between the two existing holes, where it will be covered after reinstalling everything. That hole could also serve as the point of a small spot weld to retain the plate once you had it back in proper position.
I'm just thinking off the top of my head, and I'm sure others will chime in with their possible solutions.
Good luck with this.
Larry
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