Answer
This is a confirmed platform limitation, not a formatting problem — BLOB, CLOB, GIS, and File-type attributes aren't supported for direct read/write in workflows at all, except in cases where you're calling a projection action specifically designed to manipulate that kind of data behind the scenes; no amount of escaping, encoding, or reformatting the value will make a workflow set a CLOB-type attribute directly, since the datatype itself isn't supported at the workflow-engine level.