Note: if/when the procedure below will be successful, we'll have a 4.4.x kernel with Xenomai 3, so the Bela software will NOT run on it, but at least we can start working on it.
Note2: following instructions in this post will brick your board. Please only follow them if you know what you are doing (or want to learn in the process).
I managed to compile RCN's 4.4.x:
procedure (on a Linux computer. It would probably work but take forever on a BeagleBone (assuming you have enough space available):
git clone https://github.com/RobertCNelson/ti-linux-kernel-dev
cd ti-linux-kernel-dev/
git checkout origin/ti-linux-xenomai-4.4.y -b tmp
./build_deb.sh
If the above fails, try the workaround I described here.
You should then be able to get the 4 resulting deb files from this archive.
You can then install them on the board by copying them over and running
dpkg -i <filename>
for each of them.
Good. If you do this on a BBB or BBGW, they will not boot (have not tested the BBG). I guess it's time to remove the Bela cape and hook up the serial debugger.
Anyone interested, please follow and contribute to this thread. It may be one of you @thetechnobear, @AndyCap, @LiamDonovan, @giuliomoro, and/or any one else interested.