Where there's a will, there's a way!
Those are handsome critters!
Cyperus umbrella sedge would be a good one. The leaves are up on top of 14" stalks. I imagine that Spathiphyllum and other riparium-suitable aroids would be pretty distasteful, but they are also mildly toxic and I would worry about the turtles getting sick if they were to eat very much. Leather fern eventually grows into a pretty big and robust plant, so if you can get it up to size it would probably be big enough and mostly out of the way.
Is there room to slide the planters up higher on the glass? You can raise them up so that just the very bottom is in the water and then water will wick up inside. This might not look super great with the planters rather conspicuous, but you would still have nice foliage.
I'm guessing that anything that you put on trellis rafts would probably get munched and/or trampled.