I have a 20g tall acrylic tank that I tried every way but Sunday to figure out a way to add riparium planters in it... the only way I could come up with is some type of background that is the depth of the brace, thus making the planters about even with the bracing. In the end I decided that I did not want to add a background to the tank as I love the tank and didn't want to do anything that permanent to it and ended up with a custom rimless starphire tank. Another possible way is to add an acrylic box like set up against the back that is the depth of the rim and about 4 inches tall (or more) - maybe an overflow system of some sort incorporated into it? That would allow you to suction the planters to the box and span the brace. The Riparium planters are about 2 1/2 inches deep with the suction cups and 3 1/2 inches tall.
I know what you mean about wanting an acrylic tank, they are lovely for viewing and for weight.
Ps. loved you Paludarium!