Boom! Got it!
Keyboard Maestro macro posted to the Keyboard Maestro forum.
You’ll have to either edit the script to add your Instapaper username (email) and password, or add it to Keychain. There are instructions in the macro.
The Keyboard Maestro macro is two steps. The first is AppleScript which has a slightly different syntax to the example above:
tell application "NetNewsWire"
set articleUrl to the url of the current article
end tell
return articleUrl
which gets saved to a variable NNW_URL
The second is the shell script which will use $KMVAR_ NNW_URL and do all the behind-the-scenes work to connect to the Instapaper API.
I chose ⌘ + ⇧ + I (capital i) because ⌘+I was already in use by NNW. It’s also available through the Keyboard Maestro menu bar item.