See CQL Syntax. A CQL string may be used as a type-104 query, which is defined as EXTERNAL. CQL is assigned the object identifier 1.2.840.10003.16.2 for use with type-104. The EXTERNAL should be encoded as single-ASN.1-type, using the following definition.

{Z39-50 16 2} DEFINITIONS ::=
BEGIN
IMPORTS InternationalString FROM Z39-50-APDU-1995;
Cql ::= InternationalString
  END