First off let me say that I think this deserves a dedicated discussion on the mailing list or even on HN. I fear this thread will not be seen by most redis users, unless they also happen to be interested in TCL (selection bias).

However:

I really think that the right thing to do is to embed Jim Tcl, but there is a cultural barrier preventing me from doing this.

My datapoint as a happy redis user would be: Please don't. I have used TCL but don't like it. I also think there are rather few users who are versed in it and the obscure syntax would become a barrier to redis scripting gaining popularity.

My vote absolutely goes to Lua. That's still nowhere near as popular as javascript, python, ruby et al. But I think it's a much more accessible choice to people coming from the aforementioned languages (read: most people).