It's a really creative idea, very cute thing to do in nobuild areas! First a caveat: there are much better builders/scripters than me in the forums, so if someone says I'm full of it, go with whatever they say

That said:
I see one big difficulty with it: keeping the chair on the ground.
Because the chair is attached to an avatar, you could not do this if the avatar, for example, wanted to get up and dance around the chair. The chair would swing wildly around.
But, if all you wanted to do was sit or stand in poses where the attachment point didn't move, then it could be done.
The only tricky part then comes if you're using poses you buy, as opposed to poses you make. If you just play a bunch of "sit" poses out of your inventory, you notice that you are sitting in slightly different heights and rotations with many of them. So, for every sit you add to the chair, you probably want to position the chair differently, so that it LOOKS like it doesn't move (much), and like you're actually sitting on it. Note that this "repositioning" would also give you the ability, for example, to use an animated pose where you're standing with one foot on the chair, tapping your foot and smoking a cigarette. All this can be done via script.
If you are making the poses yourself, and you can make them all so that there's an attachment point that is always in the same place, then you wouldn't need to reposition the chair for every pose, even.
[In fact, you also may want to attach a different chair (maybe a stool) for some poses. That can be done with the same script, but let's get the basics down first.]
So, you really only need three things:
1. A mod/copy chair
2. Poses/animations where the chair's attachment point stays in one place during the pose (attachment points that don't move much are spine, chest and stomach)
3. A script that:
-- Puts up a menu of poses to choose from (you don't need a separate HUD, though you could do it that way)
-- Runs the animations (gets/surrenders permissions, plays and stops animations based on menu choice)
- Adjusts the attachment position whenever a new animation is played.
Very fun little project!
