Can \ could LFS output separate traction loss for the rear and front tires?

What do you mean by output, exactly? Are you simply referring to the physics simulation or are you thinking of OutSim and/or OutGauge?

As for the simulation, sure. Each tire may, individually, slip/loose traction when accelerating, braking or turning, and many parameters such as surface, tire type, tire temperature, whether the tire has dirt on it or not and so on may also result in loss of traction for each individual tire.

You can use the Forces View (F) and Tyre temperature/wear & clutch temperature display (F9) to verify this.

The Brakes section of the Advanced Setup Guide has an example of how the Forces View can be used to determine that the brake balance is set too low, forcing the rear wheels to lock up/loose traction while the front are still gripping, for example.

I understand that LFS calculates traction at each wheel based on it's physics model. How is that event output via the simtools plugin?
What I would like to be able to do is simulate traction loss as a percentage of understeer and oversteer. I want to use two sway actuators, a traditional 'traction loss' actuator at the rear of the sim rig and instead of a physical pivot at the front there is a second sway axis actuator. One could then drive the two actuators to get a range of traction loss between full understeer thru pure 'sway' at 50% each to full oversteer.