One thing which has not been looked at in this discussion is why Jimmie's first attempt to solve the problem failed. He replaced every comma in the query by its percent encoding, '%2C'. This failed because ZnClient replaced this by '%252C', i.e. it percent encoded the percent sign. In my first attempt to help him, I tried replacing the query line by:

 queryAt: 'fields' put: 'displayName,instrument,pip' urlEncoded;
but this produced the same result. The puzzling thing is why ZnClient is doing this.

In http://www.w3.org/Addressing/URL/uri-spec.html it is stated that:
" The percent sign ("%", ASCII 25 hex) is used as the escape character in the encoding scheme and is never allowed for anything else."
Further on, there are two examples of strings which are illegal as URIs because they contain percent signs which are not followed by two decodable hex characters. Hence, if ZnClient encounters a percent sign in a query string, either it is the beginning of a percent encoded character or it is illegal.