I don't think it's possible at this time...
Their localhost testing doc gives an example of how their command line works:
Command line interface
java -jar BrowserStackTunnel.jar TI3PnSeogaDEcwSyiSzm localhost,3000,0
In my experience with selenium, that doesn't leave any way of launching the selenium test.
Here's an example of how I call a selenium test:
java -Dpfile="../properties/ff-test.properties" -jar ../runselenium.jar ca.myorg.tests.HappyPathTest
I just received an email from them that promised full support for selenium sometime next month (May, 2013):
We are working on implementing full scale automated testing including Selenium and expect it to be released sometime next month. We will notify you personally once done.
Hope this helps.