Actually, I have to backpaddle on some of my previous suggestions. I would actually recommend a custom movement mode after all. I had a look at some videos on youtube and the movement is actually more physically based than I remembered. In fact, it looked so fun that I implemented a basic version myself:
I actually only needed about 50 lines of code to achieve this. I am using my own movement framework which already has a lot of utility so it’s probably a bit more work to do it in the character movement, but it’s really not difficult. The crux of the matter is that the character movement component goes to great lengths to simulate the way living things move while actually being a rigid body (a capsule). But the Tribes movement (at least the high velocity sliding part) behaves much more like an actual rigid body, meaning you move more like a fast ball or a car than a human. This means that you can use basic physics equations to simulate the movement and just slide along surfaces willy-nilly without regard for where the hit normals are pointing. If you are still interested I can share the code.