It's something that most seismologists have thought of before. Unfortunately, there is no way of knowing how much stress has already been accumulated. We also don't know nearly enough about earthquake triggering to be able to do it in a controlled fashion once, let alone repeatedly.

You'd have to do this thousands of times (log scaling, remember!), in a new location each time, to produce enough small earthquakes to relieve the stress accumulated on a locked subduction zone fault: Large earthquakes produce relatively similar magnitudes of stress drop to smaller ones; this stress drop is just spread over a much larger area. So you'd have to basically tile the fault with drill holes and then do whatever to produce earthquakes. And hope it doesn't cascade. Thousands of times.