Your question is definitely a tough one.

I like your thinking and feel that you have a reasonable argument, but of course this isn't one of those 'ideal' situations that we often wish we were in.

Examining your situation, my first instinct was that the difference in air temperature would be too small to see such a noticeable effect. Then I started thinking about the diet coke I had today at lunch. After having the soda in my glass, I added some ice and saw more foam. This didn't make sense at first since the solubility of gases is usually higher at lower temperatures. However, the carbon dioxide of soda is forced in creating a super saturated solution. So, how does the ice speedup the liberation of carbon dioxide? My next guess would be the vapor pressure- or the drop of pressure that you eluded to in your question.

To test the temperature reasoning, you can perform an experiment with different temperature glasses and measuring the volume of foam that forms. And as our case is not ideal I thought up some factors that may also effect how and why the foam forms.

It may be possible that the thin layer of frost on the glass creates a rough surface for the foam to cling giving the build up of foam an appearance of being more than just the invisible gas escaping near a smoother surface where it wouldn't cling.

The viscosity of the liquid could control the foam rate. The way the liquid is poured into the glass may have an effect on the foaming. And looking up some information about beer I found that lighter beers almost always have less foam than darker beers because of the hops that are used in production.