A search warrant can issue just as a conscious DUI suspect can be required to give a blood sample. The Supt. Ct. said that years ago in a case coming up from Calif. I believe it's: Shemel v. Calf.(or something like that). If probable cause exists, a warrant can issue. Alternatively, he can be ordered to surrender the bullet and be held in civil contempt, a contempt where he is said to hold the key to the jailhouse in his pocket, and that he has only to comply with the court's order; a mandatory injunction, and he can walk out of the lock-up. A mandatory injunction is one that says, essentially, that '' you shall not fail to do [something].''
They could establish whether or not there is a metallic object in his head with a simple metal detector. A search warrant should be easy enough to obtain.