I apologize in advance for the long ass post. Meant to type just a quick reply and then.. idk what happened
I can't say for sure how damaging it is to use your handbrake to slide, but I do have a theory:
Pulling the handbrake for just a second to initiate a slide really isn't much different than if you did a panic stop and while all of the weight was on the front wheels, your rear wheels started to lock up while the fronts continued to rotate normally. In both instances, you're getting momentary rear lockup while the front wheels continue to spin. I don't think anyone's ever blown their center diff or even worried about their transmission after a panic stop so I'm tempted to say, while it's technically bad for your diff, it might also be fine?
I've done lots of snowy parking lot donuts in all of my subarus, usually using the handbrake to initiate, and never had an immediate failure or anything like that. But I only do that a handful of times a year so more prolonged use might be more damaging.
I remember reading about an STi group N hydraulic handbrake that included a switch to unlock the center diff while the handbrake was pulled (I think the regular handbrake in an STi does this too but not sure). That was meant for a DCCD transmission though which has an electronic and mechanical center diff, so the switch would override and unlock the electric mechanism but there would still be a mechanical connection in place. Does this mean it's okay for the handbrake to be pulled with a purely mechanical center diff like the 5 speeds have? Iunno
I think handbrake initiations in a rally car are probably like clutch kicks in a drift car; a necessary evil. Yes it's hard on the car, but for a track car some parts that normally last tens of thousands of miles become consumables. If you're still doing rallycross, I would ask the other drivers at the next event and get their thoughts, they have way more first hand experience than us forum dwellers.
and lastly..
It's hard to maintain a slide in an awd car; as you give more throttle (like you would do in a rwd car) the front end wants to pull the car back to straight. What you need for longer slides is more momentum and initiation speed. You have to think a lot closer to how you'd drive a fwd car than how you'd drive rwd. There's no good way to modulate the back end of the car mid-slide except for pulling the handbrake over and over again, so you need to have enough momentum when you initiate that it's going to carry you the full length of the slide and not run out of steam midway through. This means it can be super easy to overshoot a corner, so I try to think of it like this: There's a "window" where you enter a slide with enough speed that if you don't do anything else, you're going to fly off track. But, if you do get the car facing the corner exit and apply throttle, you can pull yourself back on track. The task at hand is to put yourself within that window, and then apply enough throttle to hold the slide.
Someone with real track experience can probably give a better explanation. I am but a lowly rural backroads guy.