Hi.
Personally, I would temporarily pose/skin a sitting HIK rig to the character.
[The skinning could be rough and dosn't have to be perfect, as you'll be doing corrective shaping at a later stage]
Then do your best to pose the character to a T-pose.
This without a doubt, would require alot of corrective shaping afterwards.
So at that point, freeze the history and delete the skeleton, the character will retain the T-pose pose without the rig.
Fix the shape of the character to a good T pose by adjusting the vertices in soft select/lattice/other deformers.
At this point, I would personally look into fixing one side and make a symmetry when done.
Freeze history and then recreate an HIK skeleton to make a final skin.
Manny Papamanos
StingRay | MotionBuilder | Maya | Mudbox
Games QA Specialist