You cannot currently specify a cert/key (or pfx + password) to pass through to the underlying https connection. This pull request adds this support, at least as far back as Node 0.6 (I don't have a running environment to test prior versions). Due to some old Node 0.4 compatibility code in there (with regards to using a custom Agent) I wasn't sure how to meld the two together.
Client certificate support for wss client
Fix some issues with assignment
Fix up a bug
Fix options defaults
Have a few updates to make.
Any update on this? It doesn't look like this was merged and I don't see any other signs of client SSL cert support yet. Would be a great feature to add.