How do I refresh/rescan the SD memory in Android 4.4 KitKat
It won't work to detect deleted files, but I just put an app up on the Play Store that will work on new and updated files: https://play.google.com/store/apps/details?id=com.gmail.jerickson314.sdscanner

It uses a workaround inspired by Dan Hulme's answer: by using the API that is only for a specific file, but manually generating a recursive list of files on the card.  This requires only the read permission on the SD card, not root, and has the nice side effect of allowing a reliable progress indicator.