Softbody tires/mud sim in Spintires are custom code, afaik, and not Havok.
Except for custom tires, you can build complex constrained vehicles (http://www.youtube.com/watch?v=D8zQQ2Rm0ow) and deformable terrain (http://www.youtube.com/watch?v=bLIBE9bsfn8) with PhysX.
a) BeamNG soft-body tech was in development for many years (including Rigs of Rods lifespan), and it is very specific and vehicle oriented. I doubt a general soft body plug-in will give you what you want.
b) BeamNG-style destruction does not fit many games. BurnOut style game with instant respawn or ultra-hardcore racing game - yes, other - no.
And this “something” will be ?