Dart frogs rarely drown in water features -- at least, that not the main reason they're not a good idea (though the concern is often used as a sort of red herring in discussions). If there is water in there, whether it is running or not, the frogs will be likely to deposit tads in it, and that could/will turn out badly. Returning pathogen- and nitrogenous waste laden water to anywhere the frogs might contact it is also to be avoided, for hopefully obvious reasons.
If you can work in more climbing opportunities -- horizontal/angled larger/thicker hardscape elements get a lot of use from leucs -- that would be good. Leucs are strong and active climbers.
