I work in an area that has a high concentration of Native Americans and we deal with this issue a great deal. I will perform the best assessment I am capable of under an "intoxicated matrix" but it still comes down to "reasonable and prudent action", the pts legal rights, and the vast gray area in between. Most often the pts are taken to a detox, or jail, but I do not permit the pt to sign his own refusal. I will have law enforcement sign and explain that because they are impaired, we need a witness to show they are refusing, other than our own crew or first responders (legal issue again). I have yet to have a tribal PD officer or the other local law enforcement (SO, SP) refuse to sign for us as they understand the need to free up a crew if they are not needed on an obviously uninjured but intoxicated pt. They can always call us, or another agency once out of our AO, if something goes wrong in transit to the detox or jail. If they are obviously injured but adamantly refusing anyway, our PD will take them into "custody" so we can treat them and then they work out the rest at the hospital.
The key to this part of the "intoxication matrix" working is having a good working relationship with your local law enforcement. Often, just explaining how we as medical responders are impacted legally is all it takes to get them to understand. You know...communicate!