Hmm. I don't know. I'm not sure there's a valid way to adjust the path that Casper Admin captures to make it work for any account. I doubt you can use the ~ syntax to replace the home path in that.
You may be better off looking at something like dockutil for this. It may be able to handle it better.
Either that, or move the file into a more generic location, like /Users/Shared/ and capture it from that path. That way, as long as said file is deployed to your Macs into /Users/Shared/ it should work to send a Dock item with that path.
