well, instead of a solution all i can offer is advice on a place to look for one*. while browsing through the Net::Ping source, i noticed that it uses setsockopt() to bind to a specific interface, so maybe you could build up your own stack based on that to act as a foundation for a telnet session that you can then use to connect to, say, port 80 on google.com to see if you're up and working on a given interface. this would also have the added advantage of not having to run as root.

* not for lack of trying, but my eyes start to bleed when i start dealing with sockets and their crazier options, i'm just way to lazy.