What is the proper way to blend between two Mecanim States in a Layer, when both animation clips have different Masks?

Thanks for the help in advance. This is the first time when rigorous Google and Unity forum searching has not given me any kind of answer.

The Scenario:

I have two Layers in Mecanim. Layer 0 contains all everyday full-body animations (idle, running, dodging, etc). Layer 1 is an Override layer which has the things that appear over top of a character, which overrides all or some of the animations underneath (blocking, hand gestures, etc). Some clips like blocking have leg motions masked out so you can still run. Others, like hand motions, have all but the arm masked out so you can wave to friends while doing anything else.

The Problem:

During transitions between two states in Layer 1 (Override Mask Layer), and ONLY during transitions, any parts of my character armature that use a Mask on EITHER state where the masks don't identically match...slowly transition into the Base Pose (T-Pose) of the armature. When the transition completes, the character snaps to his properly masked pose.

So, an example: if a Hand Wave animation has everything masked out but the arm/hand/fingers, and I try to transition into a base Idle pose with nothing masked, everything but the arm will snap to Base Pose (T-Pose) and transition into the Idle from there. The other way around (Idle->Wave), the character will transition from Idle to T-Pose (except the arm, again), then when the transition ends, snap to where he ought to be in his Hand Wave animation.

Keep in mind, this only occurs when two animation clips do not have the same parts masked. Any parts that are both either MASKED or UNMASKED do what they ought to.

Also, I cannot use Avatar Masks because in [Override] Layer 1, the same parts won't always be masked. Hence the problem.

The Solution?

I understand that you can't transition from a keyframe to an absent keyframe, if that's what masks actually are. Yet when the transition completes, the character's parts are in their proper place, so I know that works. How can I transition into that proper place?