hmm...interesting concept...assuming that you use the CPAP for its intended purpose as well as this experiment I'd think you should acquire an additional hose...as far as reducing the hose down to 3/8" I think it may be possible using something like one of those reducer hoses used for dishwasher connections and then possibly some brass fittings and hose clamps (or order stainless)...damnit now I'm going to spend my weekend going to hardware stores to see what parts would be needed
Another question would be whether the pressure from the CPAP is providing enough aeration (or perhaps too much).
I say it is worth a shot at least...
"Boredom leads to tinkering. Tinkering leads to Rube Goldberg like brewing setups." - Yoda on homebrewing. (thanks to DarthBeer)