But the runtime is at most 80 min (nominally 74) so that limits the practical amount.

It would have been nice if LP modes were available in Red Book CDs (no compression but lower bit depth and/or mono). The motor controller can already handle a wide range of speeds to maintain a constant linear velocity. And if that turned out to be technically difficult, automatic two-pass mono could work - include an extra bit in the ToC for L/R channel, and play one of them first and then the other with the second half of the tracks. The result would be a double-runtime mono disc with a seek break in between two tracks around the middle (varying in length between players) but that would be acceptable for audiobooks. Of course CD-ROMs with MP3s/AACs are available but player compatibility is still problematic.