What about enclose the lead wires in a very small tube (1/8th inch tubing maybe, steel of course, depending on the size of the leads) that is long enough to reach to the back side of the grate but not so long as to foul the stoker. A slight uphill bend would also help. Hold in place with a self tapping screw in one of the existing grate holes near the back of the grate (a small tab would help).This will allow you to run the wires up though the bottom of the hopper then drill through the side of the hopper and make your connection; a rubber grommet would protect the wires from chafing on the holes edges. I would rather drill my hopper than drill my grate. A hole in the hopper would be much easier to drill and if I make a mistake won't destroy my grate. This would also make it easier to replace if there is a need for replacement. This kind of setup would pose minimum risk to the grates and would not impact the flow of coal onto the grates.