There is an array of browsers listed in the URL browser option list, but Arc is not yet one of them. Any chance it could be added?
Alternatively (or in addition), it would be nice to see better instructions for formulating your own custom open command. Right now, you're mostly left to figure it out on your own. Including additional details would be helpful, such as (1) that "com.apple.safari" thing is called an application's "bundle ID" and (2) how to find that bundle ID for your unlisted browser.