Hello Nazarene,
SAP Queries are mainly meant for functional people to make reports without knowing about ABAP. However Infosets (SQ02) are equipped with ABAP coding sections for requirements go out of standard limits of this tool, where we Add Additional fields, tables etc. Sometimes in the coding sections (Extras), we might need to declare tables types and internal tables. You present requirement is one among such. You want to extract Longtext from a field. In my view this is a pure ABAP exercise to be carried-out in your infoset. So if you have a query already, take help of your ABAPer to fulfil your requirement.
Your specific request about Component longtext falls in the same category.
KJogeswaraRao