How do I unlock a .plist file in /private/var/db/receipts so that I can change it?
You probably need to add an ACL to the immediately enclosing folder too. 

Not certain about the entire path, but you could always later remove with
sudo chmod -R -N /path… at the highest directory level you reached to enable it, which will remove ACLs from the entire directory tree at &amp; below your /path...