You can automatically create time-lapses of your work in The Source Filmmaker by using the Screencast feature on the Help menu. This will automatically save a .PNG file inside the target Screencast folder every time you release the mouse or a hotkey. Any video encoder can be used to make a movie from the image sequence, though Valve tends to use FFMPEG, which can also be found as part of WinFF package.
Warning: This is a prototype feature that will fill your hard drive with images over time if you do not manage the folders and delete the image sequences manually.
Here's a snapshot of some typical settings, but they can be altered as needed:
Here are the same FFMPEG settings for copy and paste:
-y -loglevel panic -sameq -f image2 -i %9d.png -vcodec mpeg4 -aspect 16:9 -s 1280x720 -r 23.976 -vb 8000000 <FULL_MOVIE_PATH>