This is something interesting I noticed over the weekend. One of my beers has that distinct green apple smell to it. (I had to keg it early because I needed the fermentor. LHBS accidentally sold me a ported better bottle...) While I was using google to figure out how to spell "acetaldehyde" again, I noticed that it's boiling point is listed as being barely above 68F.
Is it possible to do a warm rest above 68 F after fermentation to effectively drive off acetaldehyde?
I've never tried heating beer, although given that commercial beer is pasteurized at around 165F/74C - far above your 20.2C/68F it may work. I'm not a chemist, but my understanding is that the 68F boiling point is for the pure substance - once dissolved in water the boiling point will change, the same as when salt is added to water changes the freezing and boiling point. If you're unsure, simply try it on a glass of beer. Put it in a warm water bath for a day and see how the flavor is. Then cool and sample next to a control glass from the keg at the same temperature.
In your position, I would simply leave it for a few months. The yeast will break down the acetaldehyde given time. It may take a long time - my first pilsener had the same problem - I just left it for nine months, through a long norwegian winter where the garage was around -1C and the beer came out fantastic.
If you want to speed it up, you could pitch a sachet of neutral flocculating yeast, like US-05 (this will also work for lager if you bring the temperature up to 60F.) The fresh yeast will absorb the acetaldehyde.
