re: sys.spawn
Looks like it will work fine, but I don't really understand it.
At this point, what I see is
sys.spawn(command,{command_arguments?},something,something,something) =o)

I'm curious how one learns to use a software that has no website or documentation =o\

Anyway, this seems to be working for my current project:
sys.spawn(player, {dir.."/"..box:text(box:value())})
I still don't know when it's appropriate to use assert(), although the example used it.

But now I have to figure out how to keep the murgaLua window hidden while the player runs, since it apparently backgrounds the process by default. Maybe that's where one of the "something" parameters comes in?
I would use iconize() rather than hide(), but my window manager does not support minimizing.
Then again, over 99% of the world's desktop computer users do not have this issue, so maybe I should just go with iconize() and only use hide() for my personal copy.