I use hydroton as a false bottom, but I leave a corner of the tank bare bare so a little pool of the drained water is available to the frogs. A little river rock wall works as a barrier for this:

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This way the water is easily accessible for draining. The water in the pool will evaporate (adding humidity back to the tank), so siphoning it out isn't often necessary. It also makes a very large water source possible (when filled up to just below the substrate) should I have to leave for a couple of days. The downsides to this approach are mainly cosmetic- the water can turn a little brown from running through the substrate and the constant evaporation will add some hard water stains.

The construction is straightforward, build a 'porous dam' out of the river rocks to make a pool area, holding them together with a little dab of silicone where the rocks contact one another to help hold them in place. Make sure there are no holes large enough for the hydroton to slip through one way, or your frog to slip through the other. Add an inch or more of hydroton. Lay down landscaping fabric (or screen mesh, but I prefer the fabric), then put the substrate on top. Live plants are encouraged to help maintain humidity, plus they look nice imo.

To increase the drainage of your substrate, you can mix in sharp sand, or add in some orchid bark, or use the full on ABG mix mentioned above. Whatever additives you use for the substrate, if the frog can get to it make sure the pieces are either too large to be swallowed or too small to cause a problem if they are swallowed by mistake.