Note that I revised the sixth point below, to add clarification after further feedback from Malcolm.

So it took me over 3 months to get around to concluding this, and a ton of help from my new friend Malcolm Holt, but yes, what I want to do is possible.

First of all, forget the throttle->aux and rudder mixes. The basic idea of what's needed is that we're setting the Rudder stick (J4) as the trim for the throttle.

• Set the Function for Channel 3 (J3) MOT, as this makes it easier to set up a safety off switch for the electric motors.
• While in the Function screen, set the trim for the MOT channel as J4 (which should be defined as the Rudder channel control).
• From the resulting H/W Select screen, set the Rate to a reasonable value (I started with +50% and ended up with +30% after experimentation), set the Mode to ATL and change the ATL from NORMAL to REVERSE. Depending on the way the motors are attached to the receiver, the adjustment direction may be backwards; in that case change the Rate to - instead of +.
• Since when the throttle is set at wide open, a rudder adjustment could drive the channel above 100%, go to the End Point menu, and set the Throttle channel high Limit Point to 100 (I believe it defaults to 130).
• Go back to the Function menu, and assuming you have the other wing's motors connected to AUX1, set up the Trim on that channel (I used 7) to be connected to J4 as for the MOT channel above, except the Rate should be the opposite sign (+ vs. -).
• Now the Rudder stick should proportionally (Not additively) affect the Throttle setting, slowing the engines on the wing you are yawing towards, and speeding up the ones on the wing you're yawing away from, but as a percentage of the original Throttle setting. If you're like me and you don't want a rudder movement to cause motors to stop completely, you will need to play with the Rates above, and then go to the End Point menu and lower the percentage of the MOT and AUX1 channels low end Travel Rates (I ended up with 85% on each). Note that I'm talking about the stick Travel Rate values, not the Limit Points. The way I did this was to set the Throttle stick at the first detente click above the bottom, then adjust the MOT low end Travel Rate until the motors stopped, and then back up one count (motors running). Then, still on the first click, I pulled the Rudder all the way over to the side to the side that reduces the MOT motors speed, and if they stop, adjust the low end Travel Rate until they just start again. Then repeat these steps for the AUX1 channel. When you're through, the when the throttle stick is set to the bottom, all the motors should always be stopped, and when at click 1 or above, the motors should all always be running, regardless of the Rudder stick position.
• Now for safety switch. Go to the Motor menu (from the Model menu), Change the ACT from Inh, and change the sw to whatever switch you want to use (I always use SF for Throttle Cut, so I used it here for the Safety). Go down to the motor off value, make sure the throttle stick is at the bottom position, and press and hold RTN to set the Safety throttle position (should end up at 0).
• Create a Prog Mix with both Master and Slave set to AUX1. Activate it and set the activation switch to the same one you used on the MOT channel (I used SF). On screen 1/2 set both values at the top to –100 to counteract the effect of moving the stick and set the Y Offset to –100 to shut the motor off.

Now, when SF is On, the motors are live and the throttle will work; when it is Off, the motors are safed. If the switch is On when the radio is powered on, an alarm sounds to warn you.

Most, if not all of this can be gleaned from Malcolm's Amazon Kindle Futaba 14SG Programming book, available here. Again, I would have taken much longer to figure this all out without the direct support of Malcolm, who has my gratitude.