What sort of lunatic advises on avoiding engine braking! I mean seriously. Avoid "engine braking" to save your drive splines. How are you supposed to ride the bike? Every time you get off the throttle you pull the clutch in?
Docc your bike has done a lot of miles, hows the drive splines look? How many have you needed to replace?
This advice is up there with not blipping the throttle for downshifts. Maintenance and lubrication is the issue not engine braking which in fact is a minor load on the splines compared to the loads from accelerating the mass of the bike.
As for drilling holes in the cush rubbers, well thats a stick your finger in the air and have a guess backyard hackers solution. It's not hard to measure the shore hardness of rubber and if you need something less hard then you can buy liquid Polyurethane in the hardness you desire make a simple mould and mould what you "think" you need.
Phil