If you did it just right, you could install the same manual decompressor that the Iron Barrel bikes use. It goes in that same place where you are talking about drilling that hole. And the Iron Barrels already have a left side switch cluster that already has the decompressor lever built in to it.
And if you did it just right, you could drill the little vent outlet for the decompressor that vents into the exhaust port, just like the Iron Barrel does.