Individual quickly settled his claim pro se with tortfeasor's carrier, then discovered he really was significantly injured after all. Now wants to know if there's anything I can do for him, despite the release he signed.

Answer: Read: Holmes v. MKTRR Co., 1978 OK 6, 574 P.2d 297: Error to grant summary judgment for Defendant where evidence indicates release entered into contemplated full recovery, while fact was injuries were permanent and serious.

Campbell, James K., Torts: Avoidance of Releases of Liability for Personal Injury, 49 OBJ 882 (1978)

These authorities are a bit dated but the Bar Journal article is pretty comprehensive and when brought up to date will, I think, answer your questions.