R.M.
I would simply ask your MIL to add you to the list so that you can take your FIL out without having to bother her for permission. It is a logical request and I don't know why she would object. It sounds to me like there is a lot of animosity between your MIL and your husband...and I am sorry that is the case. As one of the other people said ...I see nothing wrong with her asking her husband to make sure she was on the title to the house before he went in for very serious surgery. In fact it could have even been HIS idea!!! As to her keeping him "drugged up"...she may have been so overwhelmed that it seemed like the only way she could deal with it. All of us make the decisions that we feel like we have to make when we are in a situation like that, and I know it is hard not to judge when we are not actually living in the situation.
You might be able to take a lot of stress out of the situation if you could somehow repair the relationship between the two "sides" it would only make life more pleasant for everyone...including your FIL