Thanks. Today on the bus, I realized I could put substrate in the tank, it would solve the issue of water channeling under the sponge, be easier and cheaper then installing braces, as well as making it easier to repurpose the sump tank in the future.
I'm not too concerned with trapping particulate matter with the mattenfilter, I figure there are other/better ways to do that if it becomes an issue in the tank.
Mostly, with what I'm currently planning, I felt the narrow dimension of a 10G sump would restrict me to a really low flow rate. I was hoping that if I could place a mattenfilter diagonally (or maybe lengthwise as Hoppy suggested), I could go with a higher flow rate.
I just suspected there was some dreadfully horrible and obvious reason not to do this, and that I had somehow overlooked it. Eventually I figured it was worth the risk (displaying the result of hours of my click-dragging shapes in an attempt to illustrate my plans) to see what sorts of dreadful things I failed to notice.