It's been done before...

Scotland recently legislated to put DBS (deep brain stimulation) in the same category as psychosurgery, making it a treatment that can only be given with a person's consent. England and Wales have done nothing, although experiments in the treatment of depression and OCD (obsessive compulsive disorder) are starting at the University of Bristol and Queen Elizabeth's Hospital, Welwyn Garden City respectively. In the absence of specific legislation DBS could be carried out on people without their consent or even a "second opinion" from a Mental Health Act Commission psychiatrist.

The treatment must therefore fall within the scope of section 63, for which neither consent nor a second opinion is required. Even though we doubt that any doctor would, in practice, implant electrodes in a detained patient without that patient's consent, this would seem to make such action theorectically legal, if perhaps particularly vulnerable to challenge under human rights law" (Mental Health Act Commission Tenth Biennial Report 2003).

The Mental Health Act Commission may consider the use of DBS without consent unlikely, but it has been done before.

In a 1980 article Robert G Heath of Tulane University in New Orleans and co-authors described the case of an anorexic patient who was opposed to the operation: "Initially, she refused to wear the pacemaker, and restraints were required to keep it operating. During the past year, however, she has showed gradual change. Compulsive rituals have been significantly reduced, and she states she now wants to wear the pacemaker because it makes her feel pleasant and relaxed." (R G Heath, R C Llewellyn and A M Rouchell,The cerebellar pacemaker for intractable behavioural disorders and epilepsy: follow-up report, Biological Psychiatry vol 15, no 2, 243-256) However, she still had to be tube-fed and was described by the authors as only "minimally improved".
Read the article here

And more about Robert Heath's experiments here.