Cutting centered hole in mirrored object with subsurf My problem, as you can see on the image below, is basically that the subsurf interpolation of the centered hole in that mirrored plane does not transform into a circle as intended.

I know that this is completely correct, since it just takes into account the additional vertex on the mirror axis, but is there any clever way to fix or work around this except fiddling around with the vertices manually?
A quad won't give you a real circle anyway. If you look at your example, the outer holes are still bulging a little in the corners. That's due to the algorithm and can't be helped. Six or 8 sides are much more precise. So, since you have 4 (6 with mirroring) verts in the middle, turn it into a hexagon or use 8 to have a nice clean result. Depending on what you want to do with the hole, an additional facering around the holes can be helpful as well as makeing a nice and undistorted bevel later.