Hi Neil,
I've not seen this done with mojoPortal so far, but it should be possible to use the User Sign In Event Handler to code your own solution. You may also find these two user.config keys to be helpful:
<add key="LoginPageRelativeUrl" value="/Secure/Login.aspx" />
<add key="PageToRedirectToAfterSignIn" value=""/>
Hope this helps!
- Isaac