lower bitrates for speech (in opus)

opus has fine speech quality at bitrates at ~16 kb/s, the currently lowest available setting is ~48 kb/s.
as audiobooks and podcasts would be my primary conversion target, my suggestions would be the presets "Speech, High" (~24 kb/s), "Speech, Normal" (~16 kb/s) and "Speech, Low" (~12 kb/s).
i also usually increase the frame size in my low bitrate files ("--framesize 60") which may improve encoding efficiency at these bitrates.
a custom bitrate/arguments option setting would enable this, of course. but maybe a preset (or presets) would be appropriate as well.