I think the pulse rate of the tach signal is higher than the mouse button can take. The mouse is designed to take human-speed clicking.
You may wish to check out the fusion brain, a product developed here by some forum members. It's a long thread, but somewhere in there they may say if the fusion brain can accept pulse inputs such as the tach. Plus they have open source software so you can interface it with your OpenGL gauges.
If that one can't do it, then there are several other products out on the market that can accept pulse inputs and store up a count. Several are USB based, others are serial based.
I like your thinking, though. Using what you have readily available. Very clever! Find out what the maximum number of clicks per second a mouse can take and check it against your tach's maximum pulses per second.