Solution until MP4 Export is available
I just downloaded a program called Screencast O Matic which is a fantastic solution until SL has an export to MP4 feature.
You have to buy the 'pro' version for $15*, but it captures your output using the internal audio feature (i.e. you have the option to capture external microphones with the 'narration' on or off feature.)
The default output format is MP4. Very pleased with the quality and intuitiveness of this little application.
*Note: Subscription based, annual, auto-renew - careful to cancel if SL export becomes available