This little project is finally coming together. Here are the parts I settled on. The button was going to be an arcade-style button, but I was running into missed taps, which, when rapid firing Galaga swarms, might be OK, but when trying to time out BPM is a killer. I ended up using an Adafruit momentary capacitive touch sensor instead. For the LCD display (which was totally useful for debugging), a cheap ebay I2C lcd module was used.
The software is fairly straight forward except for the totally and terribly hacked I2C LCD bits which were ripped from various spots online. Here’s the code… Someday, I’ll try and post a schematic - particularly if someone stumbles by and is interested.