Yeah, something like that might work too, although it can be kind of invasive in the sense that now you need to check if people are living in a place. In Italy, that meant that the police send someone around for a look inside the house where you say you recently moved to. In theory, at least - they don't always.
I guess it's easier in countries with some kind of address register. Denmark doesn't police it with physical police, just through the address register: you have an official principle residence, and can only list one. So by definition any other property you own is not owner-occupied. (At least in the cities, I think second homes outside the cities are handled differently.)
You can get away with some cheating there, e.g. a couple who live together can claim to be living separately and rent the second one out under the table, or someone who lives with their parents can be dabbling in under-the-table real estate with an investment property they don't really live in. But it at least keeps it to that kind of small-scale thing; the couple can pretend to owner-occupy two properties when they really live in one, but they can't do it with five.
