For now, I have given up on finding a T-beam, and I will be making my own. It will consist of a 120 mm (4.7") wide and 10 mm (0.4") thick flat bar of AlMgSi0,5, also known as 6063 Aluminium, which will act as the top flange of the t-beam. The vertical part of the T will be a 30 x 10 mm (1.2" x 0.4") AlZnMgCu 1,5 (aka 7075 aluminium).
6063 is a fairly soft aluminium which should make it fairly easy to shape the neck, make holes for the pickups and make a recess where the bridge will be (otherwise the bridge will be too high). I've heard that it is so soft that it can be difficult and "gummy" to shape, but I could't get a piece of 6061 Aluminium, which was my preferred alloy. We shall see how much of a problem it'll pose.
7075, on the other hand, is very hard and strong, which is nice because the vertical flange will have to withstand the pull of the strings. It will be more difficult to drill, saw and file, but fortunately, the shaping needed on this bar is much simpler than on the top one.
I plan to join the two bars with screws and two component epoxy adhesive.
I ordered the bars yesterday, so now it should only be a matter of days before I can get going. All in all, I'm quite satisfied with the solution. I'd have preferred a "real" T-beam, but this solution allows me to have a stronger alloy where it really matters: The bottom flange.