Scripting on a running Arduino

I am looking into making scripts for my Arduino that it could download and then run without restarting the Arduino. I have in the past used the LUA library for this task, however I am wandering whether even the Arduino Mega 2560 will be able to run it? Does anyone here have experience with scripting like this on the Arduino or a suggestion for a different approach?

There seems to be another posting asking the same thing. Here was one recommendation made:

http://arduino.cc/playground/Code/Bitlash

There are two scripting-based approaches for the Arduino I've seen so far:

-- The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected