Hi Andy,
There’s no way to change the built in Open Link action but you could use the following extension:
#popclip
name: Open in Velja
icon: bundle:search
requirements: [url]
javascript: popclip.openUrl(popclip.input.data.urls[0], { app: "com.sindresorhus.Velja" });
(The above block is an extension snippet — select it then click “Install Extension” in PopClip.)
It has the same icon as the default one so you can disable the default one and put this in its place.