Letting scripts set avatar's armature parameters

Has there been any reason proposed before why this couldn't be done? So that scripts could provide dynamic animations?

I made an attempt at this - by rezzing invisible, physical, rings around each limb and trying to move them around to force the av into script-adjustable poses - but it didn't work and I don't know if it's viable.

Avatars do not exist in the physically simulated world at all. No matter what shape or how complex your avatar is, or how it's positioned, the physics engine only operates on the invisible compass-aligned rectangle of your bounding box. What you see, your animated movements, attachments (no matter how large or complex), clothes and skin, arms and legs and head, these are purely visual effects rendered in the Second Life application.

some times I wounder why scripts can't generate the contents of an animation, and get it treated as such. But I would rather have the ablity to set avtar pramaters. Such as tatto or hair color.

Yeh. A BVH file is basically a description of the bones in your av, followed by a list of angles and times for each of the joints. Since the first part has to pretty much always be the same, it should be possible to build up a list of motions and have SL treat them as an animation.