I have been thinking about physical computing, namely do something in the virtual world and a real world device changes state and similar for the other way around. See http://en.wikipedia.org/wiki/Physical_computing for more explanation.
As an example, I'd like to use an Arduino to illuminate a LED (light) when a player punches a particular door at which point the Minetest server sends a signal via a serial port to the external advice, eg Arduino. The Arduino then toggles a light. In the real world, User may then toggle a switch which sends a signal back to the minetest server which could unlock/open the door.
Reading online, Lua can support access to peripherals such as serial ports but not sure how to "hook" into Minetest. I'd like to avoid having to recompile minetest-server if possible.
Typical Architecture could be as follows
Minetest Server -> Custom Lua Mod code -> Serial Port data -> Arduino
Server: Debian Linux "Jessie" (Currently in testing) running in a virtual machine on Windows8. I can't change this atm.