You will have to work this out with the hospital. Almost every hospital has people who are notary publics working for them. With your gentle persistence the hospital can get any two staff members as witnesses, and when he is lucid call up the notary public and any two witnesses, let your BIL make the statement, and have the witnesses sign the already prepared papers in front of the notary. This will have to be at a time when the therapy can be done.
Yes, the logistics are difficult, set up by a very thoughtless legislative decision, but the only other choice is to take him to a neighboring state for the treatment.
Two-thirds of bipolar
patients initially have periods of lucidity between mood fluctuations, whereas one third are continuously symptomatic. He is in the latter group.
You have no options other than to be ready to spring into action by scheduling ECT on a regular basis, with the cooperation of the hospital, get them to facilitate this as best as possible.
If the will not try their utmost to accommodate him, then you do have the option of going to another facility or to another state. The only other option is not to give him the treatment, and that is not what you want.
If you want positive results you must be relentlessly persistent until you achieve your goal.
I wish you great perseverance and success and to that end will keep your brother in law in my prayers.