That’s brilliant, thanks for sharing!
Just to add that as of the most recent PopClip version, Open URL actions now also support an alternate url property to support this common scenario. So your extension can be simplified, if you want, to:
# popclip
name: Reddit Search | ⌥ for Reddit.com Search
icon: iconify:flat-color-icons:reddit
requirements: [text, "!url"]
url: https://www.google.com/search?q=***+reddit
alternate url: https://www.reddit.com/search/?q=***
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)