Your idea might help a bit but I think you'd still have a good amount of surface agitation simply from the water coming out of the HOB onto the surface of the water on the tank -- unless you fill your water level higher so that the water doesn't have as much drop from the "out-take" onto the tank water.
How many of and/or how big are the bottles for your DIY CO2? I've seen someone go nuts with 5, half gallon bottles
. I'd say he's pushing more than my pressurized at the moment!
As for O2 levels, are you concerned for fish? I don't want to say don't worry about your fish at DIY levels, but I can say that I actually turn my HOB off during my injection period (~8 hours) to maximize my co2. But then again, I have a Koralia agitating a section of the surface just to make sure some gas exchange is going on. I'm still evaluating this part because I'm trying to test how long my tank will last. Then there are pressurized guys who just inject and don't even use an aerator or anything at night. They either go low or turn off at night.
Just try it out. Maybe just add more bottles. That has the added advantage of keeping things a bit more stable as the instability will just invite algae. I used to do DIY but I was cheap so I let it run down before I made new batches. I had a fairly strong light too so you can imagine the glorious algae plantations.
Ultimately, you want to push enough co2 to keep the HOB running, because overall agitation with o2 and co2 exchange is good throughout the entire co2 period, its supposed to keep the co2 level stable. Good
pressurized systems are the easiest to achieve this but I'd venture that theDrake and his 5 bottles must be coming close to it if it hasn't already done so.