Normally on a music keyboard this is done by the sort of switch you describe but wired differently. It is called a break before make change over switch. You have to measure the time between when it is broken and made. In effect this means turning it into two switches. The common connection is connected to ground and the other two ends both have pull up resistors to +5V and also connect into separate inputs. You time the period when both these inputs are high, that gives you the press velocity.