I found the options AVC x264 Profile Level (High10, High4: 2:2, High4: 4:4) and encoder in 10bits color. They really are not available for Avidemux or are hidden somewhere that I have not found?
In Hybrid (Selur)
(https://avidemux.org/smif/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FiPZvr.png&hash=65a97fd4b210a68cfc16c0d1884064754df9236f)
(https://avidemux.org/smif/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F9dSO9.png&hash=3d7e3caffcc176e5b378c0d2a2209b524fa8a3cf)
change your subject and make it a REQUEST. Currently it is not in avidemux.
There is already a request, but at the "wrong" place: http://www.avidemux.org/smf/index.php?topic=10048.msg54200#msg54200
All internals are in yuv420, so i dont see the point