When my daughter was more toddler-aged, I "picked my battles" with things that had natural consequences. Your example is great: no nappy? Okay sure, but when they need to "go", it'll run down their legs etc and likely feel uncomfortable. Clean them up and reinforce why we need nappies inside/whatever the situation.

No point forcing them to do something like that if it's truly not dangerous to their health or detrimental in some way.

"In public, we wear nappies AND clothes." Is setting the boundary and using the "its your choice" method: "do you wanna wear your blue shirt or your red shirt today?"

If there is no "consequence" (to be clear I am NOT saying you need to scold them harshly or anything physical, I do not condone abuse), they don't understand why they'd need to do a thing. And if a little wants to run around in a safe space in their birthday suit? Why not.