I have been facing this same issue with my 100 year old father who lives with us and we are his sole caregivers so here is what I do...
Before entering a rest room I knock loudly on the door and then open the door just a little and shout out 'is anyone in here' (or similar phrase). If no one answers I proceed to go in and quickly enter the handicap stall with my father (he is in a wheelchair). Then I situate him on the toilet and leave the bathroom for a few minutes, calling in periodically to see if he is done. I go back in, get him up and dressed and get out as quickly as possible. If there is someone in the restroom I wait until they have left and proceed as above.
There have been plenty of times where someone enters while I am in the stall getting him situated or preparing to leave. When that happens I usually know since I hear them entering. In that case I quietly stand in the stall and wait until I hear them leave before leaving myself. A very few times I didn't hear the man come in and thus open the stall door to see a man standing at the urinal. I have simply kept my head down, eyes averted, and moved out quickly. I carry hand cleaner so that I can wash up without stopping at the sink if need be.
I honestly wouldn't go through the whole 'ask an employee' scenario...it is too much trouble for you and for them. This is one of those situations where the best is to be respectful of the other men needing the facilities but to meet your husband's needs at the same time. I agree it is not feasible to wait to get someone to help when they really need to go and the option of letting them soil themselves isn't at all acceptable. I think the person who told you how you must do it might be following the 'letter of the law' (if that) but not realizing how it works in real life. The way I have been handling it for over 3 years has never put me in a bad situation or had people threatening me with the police because I entered a men's room. For the most part people (especially men in the restroom or those that have to wait) have been more than accomodating, helpful, and kind to our situation. I would think you'd find the same.
I do want to make a sign with both a hanging option and a magnet (so many doors are metal) that says 'occupied, please wait' or some such phrase. This would be very helpful not only when I leave him in a stall but when we use family restrooms since he can't get to the door to lock it and I perfer to step outside of the restroom while he is taking care of his business.
Of course, family restrooms are the best and easiest option. Many shopping malls have these in their main areas. Target has family restrooms and I've been known to make a potty stop there simply because it is so easy and convenient. Many newer buildings have them too. Maybe one day I'll write a guide to handicapped bathrooms! It is amazing how many are very user unfriendly and/or essentially unusable. My worst experience was the handicapped stall where the door opened in such a way that you couldn't get to the toilet if you rolled a wheelchair into the stall, that was a pain.
God bless you for taking care of your husband in his infirmity and working to make sure that he is able to enjoy and experience life rather than being stuck at home. Hope you continue to be able to get out together.