I am definitely having this exact problem. The smell on my bass is unmistakably that of dog urine there is no question. The bass has been in a stock molded case in the closet for something like 20 years. It has a rubbed oil finish. The case smells a little bit, but I think that’s because of the smell from the bass.
I already started doing all of the things that you’re suggesting. Surfing the web suggested that it was the Birchwood Casey gunstock oil outgassing. I don’t believe that is the case here because it would smell like Birchwood Casey gunstock oil and not dog piss. I have other instruments finished with that and I’ve never had the issue and I know the smell of oil. It’s not offensive to me.
No dog ever could’ve possibly peed on it here when it was in a case in the closet. I’m pretty sure if it smelled like that years ago when I bought it and played it of course I would’ve known.
I’m trying a little bit of nature’s miracle, pet odor neutralizer rub down with a cloth careful carefully.
I’m gonna replace the pickguard (which delaminated) and the strings and when I take it apart, I will take everything off of the headstock and work on it from there.
I’m also gonna try giving it a little time in the sun because that always helps us all feel better.
Kidding aside, keep the suggestions coming, and I will probably try all of them. Like the OP, it is not playable as it is, and it leaves the smell on me.
I’m tempted to try the naptha next.