Appreciate all the comments, both about the physical building and the relationship component.
I'm about 70% through a 36x36 barn build. Don't anticipate needing another barn/man cave/toy storage structure. And that barn will be relatively full with equipment/off-season boat storage, and NOT set up for a living space.... "It's mine I tell you! All mine!" [laughs diabolically with a crazy look in his eye...]
As for the relationship stuff: We're being really careful in communicating thoroughly with my son about this. Couching every conversation with:
It's important for you to do what you feel is best for your future. We're offering this as an option if you think it's useful. His fiancee isn't up to date on the very latest aspects of this conversation. We've included her in past conversations about deeding them some of the land to build on, a couple of years out. We're leaving it to him to loop her in on this latest wrinkle. We know she likes the idea of a tiny house, but we're not clear on whether she thinks living this close to us would be a problem. We'll let our son hash that out with her. Don't want to put her in the uncomfortable position of having to tell future in laws to go pound sand.
We don't want to control their lives, but we do want to maximize their chances of success, if they want the kind of help we can give. Ultimately, up to them.