It doesn't work unfortunately:
Exact the same thing with radeonhd. Xephyr here I come
[ 0.064357] (EE) Screen 0 deleted because of no matching config section.
[ 0.064372] (II) UnloadModule: "radeon"
[ 0.064377] (EE) Device(s) detected, but none match those in the config file.
Fatal server error:
no screens found
Please consult the The X.Org Foundation support
Please also check the log file at "/var/log/Xorg.1.log" for additional information.
ddxSigGiveUp: Closing log