How hard/easy is it to determine when a thumb file has last been accessed (by the squids or any means for that matter)?
If easy, why not have some process delete the thumbs that have not been accessed for (squid expiration time + 1 day)? That ensures thumbs live for as long as needed (or until purged), without adding to the scaler's workload. Basically let the thumb files expire as they do on the squids.
I imagine the first run would be a mammoth job, but subsequent runs shouldn't be stressing at all.