I can think of many chemical and mechanical processes that might work, but choosing one that's cheap and reliable will be a huge challenge.
For example: soaking in a mild acid, or complexing agent (e.g., EDTA?), may help to remove oxides, but there's little guarantee that the bare metal will remain pristine until soldering. Solutions could be administered to whole reels (notwithstanding their labels), but washing and drying the reel would be a huge challenge. (Needless to say, a baking cycle would be needed for MSL parts too?) It might honestly be better to de-reel, process, and re-reel the parts.
Mechanical processes like wiping, burnishing, brushing, sanding or cutting, could very easily develop huge charges, resulting in destruction by ESD. Or apply stresses to the part causing internal damage, if not cracking that's externally visible. (I would have to guess, a vibratory polisher with gentle media like nylon + walnut shells, with a water / cleaner bath, might be okay. Still have to deal with sorting, drying and taping the parts.)
Better yield may well be had, simply from heating it higher/longer, and using a slightly more active (but still not corrosive) flux. This runs the risk of damaging parts, or other parts besides the chip in question, but maybe that's worthwhile?
Tim