llSitTarget

Is it possible to dynamically change the SitTarget while an avatar is sitting?

Yes and no. You can change the sit target whenever you want, even if an avatar is sitting on it, but I think your asking if you can move the avatar via the sit target while sitting on it, to that the answer is no. So if person A sits on the sit target at postion 1, you ccan change the sit target to position 2. Person A is still at position 1, and hops off. Person B sits on the prim, Person B is now at Position 2.

Hope that made sense

Unfortunately I think it does. What I'm trying to do is load multiple sit poses into an object and rotate through them, but if I can't move the SitTarget then all the sit poses have to have the same base locations...Oh well.....

There would be a round a bout way you could do it and that is to use a pose ball and MLP:

/15/39/106798/1.html

If you just went and got the latest set of them then you could do what you are asking fairly painlessly. If you are more adventurous you could look in the scripts and see if you could find something that would help.