Maybe Lotus position would be more suitable, but possibly caricatured also.
Seating cross-legged or on knees should be valid position. Especially, for the moment, spells are casted without speech nor gestures (a nice addition would be extra parameters to cast command:
\"/cast Rock Armor speech of the invocation blah blah\"
this text being sent to server and displayed only 1/10 times to avoid saturating spam).
Enki\'s posture is stable enough to cast by concentration (and no gestures with hands).
I also don\'t think lying down would be an effective way to concentrate.
At last, about Enki sitting position, PS is a RP and RP does not limit in what is implemented. To be sitting signifies to be sitting in the mind of the player, whatever the implemented attitude. He can also accompany this action by a text (in a shortcut for example):
Shortcut 0
/sit
/me sits crossed-legs, bows his head, frowns and begins to mumble with a voice too low for an enki...
Then, the player type by hand the command he wants:
/say Another boowttle of wine, daaaarling !