As I'm collaborating with people writing LLL rather than Serpent, I've created an lll-mode for GNU Emacs:
https://github.com/robmyers/lll-modeIt's derived from lisp-mode, with added support for {} and (most) lll and EVM keywords. Enable electric-pair-mode (Emacs 24.1+) for smart paren insertion.
I hope to make the indentation for e.g. (return) better match the POC 5 lll examples eventually, but for now the indentation is more traditionally Lispy.
Bug reports and patches gratefully received. :-)