This is an alternative to
VWR-812. There seems to be a number of issues related to the use of alpha in avatar layers, and since the avatar already has a provision for beng "truncated" (the head is omitted in mouselook, something that you could only see on First Look 57575 when looking into a mirror in Mouselook), it occurs to me that it might be simpler to have a set of flags on the
shape of the avatar (or a new asset - visibility):
Omit body parts:
[] Head
[x] Right Hand
[x] Right Forearm
[x] Right Upper Arm
[] Left Hand
[] Left Forearm
[] Left Upper Arm
[] Torso (should this be further editable?)
[] Right Foot
[] Right Lower Leg
[] Right Upper Leg
[] Left Foot
[] Left Lower Leg
[] Left Upper Leg
If all the components are checked, the avatar might be represented by a "Baby Ruth" cloud that could be hidden inside a prim.
Advantages over VWR-812:
(1) No need to calculate an avatar mesh that won't be displayed.
(2) Allows for asymmetrical avatars ... the texture layers for avatars do not permit both left and right arms or legs to be hidden separately.
(3) Eliminates alpha layering issues.
(4) eliminates display weirdnesses (for example, seeing internal parts of the head through the 'hidden' back of the skull.
(5) Lower overhead to implement (needs only a few bytes).
If it's part of the Shape asset:
(6) No new asset needed.
Disadvantages:
(1) Less fine grained control.
(2) Doesn't act as a stalking-horse for improved clothing layers. 
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Alternate user interface - sliders:
[0]========================[100%] Left Arm
[0]=====---------------------------------[20%] Right Arm
[0]========================[100%] Left Leg
[0]========================[100%] Right Leg
[0]========================[100%] Head
[0]========================[100%] Torso
Advantages: