Sounds like an interesting project. 
Since I don't know your use case or purpose of wanting such a device, I just wanted to make sure you know of other options for easily triggering a number of macros, just in case they might work for you.
- Trigger Macro by Name action (KM Wiki)
- This works really well, and with a very few keystrokes you can trigger many macros
- MACRO: Execute Macro by Name
- This is a similar macro, but with a different UI
- Some prefer it to the built-in Action
- Palettes (KM Wiki)
- There are many different ways of using Palettes
- The more I use them the more I like them
- To see some examples, see Topics tagged with "palettes"
- And just for general reference, see Triggers (KM Wiki)
IAC, you could have your macros triggered by both your device buttons and one of the above.
Good luck, and let us know how it goes.