Zach,
I just don't see the basis for an assumption that there is any compensation going on. If you do military presses to failure, at some point you won't be able to lock out the last rep. This is not necessarily indicative of a lack of mobility or some kind of compensation. Same thing with a static overhead hold -- you can't hold it forever, no matter how good your mobility is. At some point, you get too fatigued to keep your elbow locked out.
If you fatigue quickly because you are using muscle tension to fight against your lack of mobility then that IS a compensation. But anyone will fatigue at some point.
The barbell TGU is not a good comparison to a KB. A perfectly vertical arm will keep the center of mass of the arm and barbell over the center of the base of support. But a KB will displace the center of mass outward, away from the base of support.
I'm NOT saying to keep the arm off vertical as a rule or cue. I'm saying that, since the KB puts the center of mass off axis, it is impossible to achieve perfect vertical alignment of the axis of the arm, the center of mass of the arm/KB, and the base of support.
Therefore, the best groove will be a compromise, albeit within a narrow zone around vertical. And that compromise will vary somewhat throughout a rep, and based on the size and weight of the KB (since a bigger KB will put the center of mass further off axis, and a heavier KB will exert more force).