Cool idea + thread! To get some more air/water interface action going on, maybe you could route an airline in through the outlet standpipe, and then back down (up and over so to speak) to one of those flexible airstone/bubble wand type of things (bent into a ring arrangement), so as to create a 'curtain' of bubbles upflowing against the inner surface of the screen. I think the air build up would still be able to escape the reactor through the outlet, but might result in some weird gurgling noises...?
Or, perhaps more simply, maybe an airline hose (w/valve + some kind of silencer) could be affixed to the pump intake to create an aspirating venturi (err... I guess this would be like a needlewheel, but just with a normal impeller), hopefully resulting in frothy/turbulent conditions.
Come to think of it.... a cheapo HOB protein skimmer could be modded fairly easily into what I'd imagine would be a pretty powerful algal turf scrubber. I'm thinking LED strip wrapped around the outside, along with maybe a submersible glass one hung in through the middle (where the foam would normally collect), such as to light both sides of a tubular screen (much like your design). Probably wouldn't want the bubble density too high though, as I'd imagine this might impede light transmittance to the screen (actually, unlike a skimmer, larger bubbles might be better in this case...).
Anyways, sorry for the long + rambling post; keep us posted on the algal harvests!