I would use Credits:Any - Is Exactly - Organ

This would in my case not find all organ players, only those entered under Musicians + instrument when they are not the main artist. It would not find those entered as the main artist (as in the case of Bach solo works, for example) when there is not actually any reference to the organ.

It is a bit of an anomaly that when a singer appears under Musician as a contributor to an opera, say, you can add soprano, alto, tenor or bass but as the main artist they are just Artist. I suppose if you wanted to be comprehensive you would enter them as Artist and also Musician, but I couldn't really be bothered to do that.