This feature has been completed and will be available in the nightlies and release 8.1.21. some notes on how to accomplish this on each platform (works for Designer Launcher, Vision Client Launcher, and Workstation):
mac:
defaults read ~/Desktop/Designer\ Launcher.app/Contents/Info CFBundleShortVersionString
linux (only works if its the first argument):
designerlauncher.sh --version
windows (powershell. Also works on the setup/install EXE file):
(Get-Item -Path 'C:\Program Files\Inductive Automation\Designer Launcher\designerlauncher.exe').VersionInfo.ProductVersion