I made hooks out of wire coat hanger to hang shower caddies in my tank.
What about bending some coat hangers so that they hang over the edge of the tank, while also supporting the mangrove?
I think I've finally found an easy way to keep the tops of the mangroves out of the water, the roots in the water and then entire plant upright. I actually used a variation of your coat hanger idea. In the garage, where I'm stilllllll storing my pipefitter ex's crap, I noticed some soldering iron (soft bendy stuff - comes in a roll - I think that's what it's called) and had a lightbulb moment. It's so easy to bend I could wrap it around enough times to keep the mangroves upright. It was pretty easy to position each one so that it held the plants securely without restricting their growth. I just took the extra and created a little hook to go over the back of the tank. The metal isn't in the water at all. They've been this way a week or so and it's working perfectly. I'll check on them periodically to make sure it's not getting to tight around the stem.
I have been using shower suction cups since I started this thread but, they fall constantly. In the packs I have been buying at Walmart it seems hit or miss as to how many in the package will have permanently dented suction cups that can't be used at all. The other problem is securing the mangroves to the suction cup's hooks in a way that doesn't allow them to end up sideways.
Just thought I'd update this with the solution that I've found that works.