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exachillus - A calibre based multi-library search would need to deal with different databases having different sets of custom columns, and different databases having custom columns with the same name, but different types and meanings.
If anything were done to implement cross library search it may have to limit itself to standard columns only. Then there's the problem of how to present the results, would need something like a hyper library view
Can't see it happening this side of Christmas
BR