FREETEMPORARY on blob with no reference
Answer originally left in comments by David Mann:

If you are unable to run FREETEMPORARY then I believe your only option is to disconnect/reconnect the session. This will cause a cleanup of any temp blob space used by that session. Had a similar issue here where we couldn't touch the app code, so we ended up tweaking their connection pool to expire and reconnect more frequently.

Oracle Metalink Doc ID 802897.1 "How to Release the Temp LOB Space and Avoid Hitting ORA-1652" covers this, for 11g your option is to invoke FREETEMPORARY or to exit session. They also mention a new event 60025 which is supposed to free temp segments when all the temp LOBs for a session are no longer used, but I didn't see any other detail on that.