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I have a "window box" type planter behind my turtle aquarium. I want to grow a couple land plants that drape over the water, something the turtle could occasionally snack on when the plant grows low enough. I was thinking something like Lysimachia nummularia, Hydrocotyle tripartita, Hydrocotyle sibthorpioides, or Marsilea crenata.

Do you have any suggestions?

I don't think anyone has tried it before, this could be a fun experiment. I'd review some of the plants here: https://www.anapsid.org/resources/edible.html

That’s a great resource, thanks for sharing it-plenty to go through there. It definitely makes it easier to narrow things down to species that are actually safe rather than just guessing.

I might try a couple of faster-growing trailing plants first and see how well they handle the occasional nibbling, then adjust from there depending on how destructive the turtle decides to be. There is also a wonderful tool that will instantly tell you what kind of plant and identify plants by photo you're looking at and whether it's suitable for your turtle's aquarium- Botan app.

Out of curiosity, has anyone here found certain “safe” plants that turtles consistently ignore versus ones they tend to demolish straight away?

Ultimately it depends on their personality. It can vary drastically between each RES, so you will have to experiment.