You can do this with a server-side scripting in DreamFactory 1.6, which is about to ship on Bitnami (1.6 is on GitHub now). The hosted developer sandbox doesn’t have scripting, so you need to install DreamFactory 1.6 from Bitnami or directly from GitHub.
On the user creation event, have a JS script that makes a call to the Twilio API. You can call Twilio directly from your script but for security, you might want to store your Twilio API creds as a remote web service in DreamFactory and call the DreamFactory service for Twilio from your script (see this post for more about remote web services).
Here’s some more info on scripting. We’re writing up a bunch of examples on the blog soon too…stay tuned.
Hope that helps. You can contact support@dreamfactory.com if you have specific questions on implementation of your script.