More precisely, I was wondering if the standards for "CD Extra" and the El-Torito extension to ISO9660 are compatible and allow to create an enhanced CD with one audio track (readable on a CD player) and one bootable track (e.g. linux installation DVD)? Has anybody already tried doing this under linux?
Yes, this is possible.
[...] Bootable Data CD supported, even a bootable Mixed Mode CD [...]
I've found several claims that K3b supports this too, but I haven't been able to verify it yet.