I don't know Entrek knives, do they have removable scales? So then, you feel that adding liners would make the handle thicker than you want it, necessitating thinning the scales before installing liners?
Or, Are you somehow thinking of cutting a recess around the edge of an installed non-removable scale to glue liner stock into?
I hope it's the first case. A belt sander will work, as long as you are careful not to remove too much or remove material unevenly. A sharp belt will be your friend here. Keeping a set of calipers near the grinder will help you keep everything under control. After grinding, it helps to lay some 120ish sandpaper on a piece of glass or polished granite and hand flatten it further. Wear a respirator.
In my opinion, if you have access to a milling machine, that's the cleanest and best way to evenly remove thickness from a piece of micarta. Make sure you have it clamped so the top surface is true/flat to the table, and use an end mill to trim it down to the thickness you want.