Customizing ones avatar programatically

Is it possible to customize ones avatar programatically, say, via a script, rather than manually?

Yes, and no.

Not with the LSL scripting language; there are no functions for doing so.

However, since the SL code is available, I'm sure it's possible to create a viewer or side-viewer that could include an auto-avvy function of some sort. It would just have to be made with something more powerful, and less in-app, than LSL.

Also, I don't see any real reason you'd want to... without some relatively powerful body comparison program to compare between a source and the avatar, there's not much advantage to doing shapes by script, especially since shapes are savable and sharable.

I wouldn't mind an "llWearItem(string inventory_name);" with PERMISSIONS_WEAR_ITEM set. Creating a shape asset from a script, though, isn't likely to happen. LL has repeatedly said that they are not going to allow LSL scripts to create assets.