Good call! The environment variable lockdown is new here and I was surprised we couldn't hit our own user env vars. If I do as you note and open Start->type "environment" -> Choose "Edit environment variables for your account" that does allow me to set NLS_LANG as my own env variable. Too many years of going to Advanced System Properties (which I can't access anymore) and I didn't even try to look for another way to get there. Of course I'd have to do as you said which is have corp admins cleanout registry for that to work since the registry settings still override that.

So that gives us 3 options for anyone else reading this thread:
1) Launch PlSql Dev form bat file where you explicitly set the NLS_LANG before launching the tool Example:
SET NLS_LANG=AMERICAN_AMERICA.UTF8
cd <your installation directory>
start /b plsqldev.exe -- the /b makes sure the command windows proceeds and exits so it doesn't hang out in the background
exit

2) Clean out registry, set your own env var for NLS_LANG

3) Update registry vars for appropriate Oracle Homes to have correct NLS_LANG. This is the simplest one if you have admin right or corp security allows it

Thanks,
Jason