It did work. I added the map-from line you suggested, restarted MediaTomb, removed the video files from the database, put them back in, and now all the .mpg files I have show as playable, and all but one do actually play.
Try putting this line in your config.xml in the <extension-mimetype> section:
<map from="mpg" to="video/mpeg"/>
If your extensions are other than .mpg. replace the mpg in the mapping to whatever the extension is. You may also have to clear the old video files from the db and re-import them (using the web ui). Let me know if this works.
I also changed virtual-layout type to "js" and purged and reimported the directory of 1280x1024 jpeg files I listed earlier, but that didn't make any difference. The same ones as before show as not displayable, and those that are displayable show up as wee little images.