NardJ:
I want to write an (bytecode) interpreter / virtual machine running on an Arduino Uno (to make larger sketches possible using external RAM but also to allow modifications on the fly by the user).
Wouldn't it make a lot more sense to use a RaspberryPI or similar?
If you want a challenge how about building some code so people can program an RPi (sans Linux) with the Arduino IDE. That would make an interesting reverse-takeover 
...R