It's a sticky wicket, Cynon- I'd love to get them in early this time around- but we'll need the actual animations and models to work with, that's like step 0 for any of it to work.
The implementation we ran with last time was basically a phenotype change- we switch the character model for one shaped like the same character riding a horse (or pony)- because of this, we get complications like the fact that the horse can't be targetted while being ridden, etc. As far as I know, no one has tried making these for NWN2 yet, last I heard making custom 3D models with full and realistic animations (yet still able to incorporate the looks of the rider properly) was pretty far out of ALFA's tech reach (certainly way beyond mine). DLA were the groundbreakers in this area for NWN1, so we'll keep a sharp eye out for their progress, wouldn't be suprised to see them carry the torch for NWN2 as well in this area.
Note also- alternative mounts like worgs will each require their own set of these models and animation sets, it's not as easy as switching the "creature type" or something. Expect horses to appear among community resources long before anything more fanciful.
If they stick with a phenotype implementation like we had before, we can probable re-use most of the scripts I wrote for NWN1, maybe even with some nifty improvements that we couldn't/didn't get in the first time around (persistent saddlebags, "loyalty", training, stealing horses, feeding, etc). If they work out something different, we'll have to see how much would be involved in getting them in. I'd definitely like to see it, though. If we can get our hands on the right starting resources, we can definitely script something workable.