Yes, that could work, though I think if you make it too easy for people to use it for free, then many will do so just as the path of least resistance, at least

Yes, that could work, though I think if you make it too easy for people to use it for free, then many will do so just as the path of least resistance, at least until you make an issue of it. Just putting some kind of minor barrier in the way is enough to make paying the easier option, and it doesn't require any monitoring or enforcement.