Have you considered a port in C of Tiny Basic? Or, perhaps rewriting the UCSD Pascal p-machine to your architecture from Z-80?
Seriously, though, JavaScript would make a good embedded scripting language, but I've no clue what the minimum memory requirements are for the VM + GC, nor how difficult to remove OS dependencies. I played with NJS a while back, which could possibly fit your needs. This one is interesting in that the compiler is written in JavaScript (self hosting).
