A Salisbury is just an LSD that can have separate coast and power
locking, if I'm not mistaken. No difference in the math there.
You're probably already doing a Salisbury without knowing it.

As for the torque bias ratio with power/coast/clutches: I'd think
there'd be a friction coefficient in there too, but maybe in GPL Dave
just put it at 1. I don't know if there was any preload in the GPL
model. From what I can remember from driving it (it's been years and
years since the last time, mind you) there didn't seem to be any, or
at least very little. I'm not entirely sure quite frankly whether
changing the number of clutches changes the preload. From page 742 of
the great Milliken's "Race Car Vehicle Dynamics", the triangular
regions are controlled by number of clutches (and their friction
coefficient I'd think) and ramp angle, while the preload area is a
pretension torque that has to be overcome separately from this and is
controlled by the springs or a washer or whatever. Increasing the
number of clutches might increase the preload, but I'm not so sure
about that.

Here's a nice way out of that conundrum for you that I'd use: What
the engineer really wants in the end is to specify the preload
*torque* at the wheels rather than the preload spring force. That's
the important part for handling (anyone know what the spring rate of
the belleville washer or whatever is providing the preload is?). You
adjust the preload spring force to whatever gets you the desired
preload torque. So if you have preload torque as the input instead of
preload spring force, you bypass this problem entirely by keeping them
seperate. If you add a clutch and want 50NM preload torque before and
after the change, then you can just keep them separate. You might
assume somebody goes in and adjusts the preload spring a bit to
maintain that torque if indeed it changes. If I change the diff's
torque bias ratio, I want to make sure I'm not changing the preload
too without intending to.