Have you tried running the arbor nut the full length of the threads to clean them up a little. Generally the threads on a TS arbor are very flat topped and thick to prevent damage when changing tight fitting blades. I would think that it would take a lot to damage them to the point that they couldn't be used.
You could put a washer on it. If it was against the flange it would throw off your fence scale and require all new ZCI. If it was placed after the blade it wouldn't be any different than a blade stabilizer.
I am sure that replacing the arbor shaft wouldn't be too difficult if you could get a replacement part and have some mechanical inclination.
Dave