I'm a waterfall guy, but submerged can work well in certain situations, and sometimes it's all you can do. The whole thing centers around water flow rates. You have to have water movement across the growth substrate and it must have some kind of speed or turbulence to it.
With a vertical waterfall, this is inherent.
With a submerged scrubber, you must make this happen. In your case, you need something causing the water to mix up near the submerged portion of the screen. You could do this with a pump shooting water up from underneath (difficult) or an airstone (less difficult) or you could make the screen slanted and place something under it (like a piece of acrylic) to force the water to run along it, like a dam spillway.
I would opt for the combo approach in this case
The portion that is underwater would have a 'river' of water flowing across it underwater; the part of the flow above the waterline would shoot down underwater to give you this flow. However, the intensity of this underwater flow will probably dissipate pretty quick if the slope is constant. So then what you might do is curve the back plate a bit so that the laminar input flow is more or less forced to follow the curve.
This curve couldn't be too drastic or else you lose the effect of light, but I can foresee starting with a flat maybe 10 degree slope from the pipe to the water level, and then a couple 10 degree bends in the material under the water line. That should do it.
So I just came up with that off the top of my head, maybe it will work, maybe not. But I think a combination of both types would be best to try if you have the space to do it.