@weedfreak - as you found you cannot set environment parameters using the plugin, only arguments to pass to the launched executable, which is not what you want.
I know next to nothing about Linux, my toes curl any time I am forced to use it. Can you not just set that environment variable in your script you use to launch calibre? Or does that LC_ALL setting interfere with calibre operation?
Another option may be to just hack the plugin code for your own purposes, just edit action.py in the zip around line 213 to do whatever you need to do.