So I take it this is not a standard ABS plastic trim that comes on an AGA/Aqueon/Marineland tank? Do you have a picture?
I've seen some really old tanks built this way, with a thick piece of glass about a foot wide siliconed in. The issue is that this is very much a structural brace and due to that, when you remove it and add is back in, you have to make sure it's done right (I believe you would want to force the front/back panels apart so that you can get silicone in the gap, which could be tricky) and then clamp it after siliconing it back in, and you would want to let it fully cure before completely filling the tank (that means, 2 weeks minimum).
You might want to call Miracles or Glass Cages or another glass tank manufacturer and see if they are willing to give you some basics procedural pointers. Don't take my advice as the way to go, that's just a logical guess on my part.