Alright i now have a rough idea how this (might) be possible to do. I've used IDA to get the signatures of said functions and created a gamedata file for them.
I dont see a problem with implementing the FinishLagCompensation as it just requires a CBasePlayer to be passed, which sdkcalls conveniently has pre-implemented, however StartLagCompensation requires a CUserCmd as the second parameter, and i've yet to find out how i am supposed to pass, or even get that correctly.
Edit: Currently trying to work around this by just using
BacktrackPlayer and giving it a self-calculated time