OK, I don't know how album gain is calculated, so I'm not sure if I'm asking something impossible, but I think it would be a quite nice feature.
Let's say I've got an album (actually there's LOTS of them..) and all the files already have track gain. Now, what I'm asking for, is that foobar could calculate album gain by just "checking" the track gain status of all the files, without having to go all the way through all the files. It would make calculating album gain really much faster in this case. Would it be possible?

No.

because you would need to have stored all the statistics from the track gain scanning in order to do so. Since all that is stored is the replaygain value you are missing the required numbers to calculate album gain. Album gain treats all the tracks as one big track, which gives you different results than averaging track gain values.