I recently did a lot of looking into this, and sadly most of the previously wide-open loopholes for rooting LG webOS were all patched in the last ~6 months. You can fiddle with dev mode but you can't get proper root.

I basically settled on an (incredibly expensive) Sony commercial Android TV -- beyond the ADB method, their commercial line gives you additional admin controls over which apps are allowed to run and which are allowed on the network. Between the two i felt I'd be pretty content.

Granted i haven't tried it because my new job fell through and a $1400 TV was no longer an option.