Hello,
It's been a while since you posted here, but maybe you'll find my response useful.
Indeed, there is no inset tool designed for non-planar quads.
Here's some workaround:
http://sketchucation.com/forums/viewtopic.php?f=323&t=39442&p=658977&hilit=inset#p658977
You can also take a look at Tools On Surface by Fredo. It has a tool called Offset On Surface. It will do half the job, but you'll still have to merge excess vertices manually.