And of course, I'll share.

The tripod kinematics have been a PITA with a whole lot of trial and error with determining how to tune everything.

But.. today it dawned on me how exactly how I am supposed to do it.

Moving the endstops has no effect with correcting curvature of the X-Y plane.  They do allow you to level the build plane though.  Home the machine.  if the end effector is not in the center when homed, move the endstops until it is.  That will get you 99% of the way there for leveling.  The last 1% you can get by measuring the height from the build plane a given distance both towards and away from an axis.

Moving the HOME position itself, vertically at least, also has zero effect on correcting the curvature of the X-Y plane.  It just affects what your G92 offsets end up being.

The real secret is this.  

1. Home the machine, then home it again.  The first homing gets the lines in the ballpark, but the endstops could still be higher/lower angle when they engage the switch at that point.  The second homing is always perfect as you are starting from a known height.
2. Measure the height at center.  I do this with a block, moving the end effector in coordinated mode to match it's height.  Then move out part way to the edge of the build area, I do this with G0 Y120.  Measure the height again.  If the end effector is higher at the edge, reduce your TRIPOD_* values.  This is assuming of course you have already leveled the build area!  If not, repeat the measurement at G0 Y-120 and then you will know.
3. Once you have the machine tracking a flat plane, then equally adjust the SCALE on your AXIS to correct for the dimensional distortion you may have introduced in step 2.

It now tracks perfectly flat throughout the build area.  Whew...

This has also uncovered an issue though.  The virtual pulley profile needs some tweaking.  At the top limits, near the edge of the build area, the effector starts to tilt away from the center slightly.  That is also where the virtual pulley is the most 'engaged'.  Luckily though, for this project, it won't matter as everything it will be doing is at the bottom of travel.