I'm not an engineer. But lets think of a pressure cooker. If heat keeps being applied, it will eventually expode. But it has a steam vent that prevents it from happening.
Why not the same concept to drill holes that vent the pressure?
So lets just ignore that sitting in a drilling rig over a volcano aspect of it.
There is _INTENSE_ pressure from below. Opening a fissure would probably result in exactly the opposite of what you're trying to do here. Once the magma started moving, it would erode the hole larger and larger until it all comes out like popping a zit. I suspect that it would happen in fractions of a second.
Be thankful that there are mountains of rock sitting on top of that magma dome.