An extension to llSitTarget that allows one to specify a number of AV points that the sit position is relative to...
FEET_RELATIVE = where the AV's feet would be when standing normally (it is up to the scripter to adjust for where the feet actually are with whatever pose they are using) - good for 'standing' on things like surfboards.
KNEE_RELATIVE - as above for kneeling on things (assuming about a 100deg bend angle at the knees)
PELVIS_RELATIVE - current situation
HEAD_RELATIVE - Where the AV's head would be if they were standing normally - good for pillows, etc.
HAND-RELATIVE - Where the AV's hands would be if held strait above their heads - for those who like hanging around in BDSM dungeons ;-P
I imagine this would be implemented by the system adding/subtracting relevant AV-height attributes from the stored prim sit position before actually placing the AV in position.
GOAL - being able to stand/lie on stuff without floating above (if you are short) or sinking feet-first into stuff (if you are tall).