I made changes to the old Dolphin YouTube iframe code to include a whitelist of allowable video sources. I basically left the Dolphin iframe in place so that mine would not get changed during any upgrades. This would be a nice addition to Cheetah that would allow a white list to be developed, easily changed from the back end instead of editing the htmlpurifier iframe file when changes would be needed. I added several video sources; vimeo, facebook, bit chute (really important one with the YouTube censorship), etc so that my members could post videos from more sources. I posted my method of this on Boonex but of course a better solution than mine might be available now. Basically it uses regex to look at the URL and if matched, then allows the iframe to be embedded.
Could you provide a link to your boonex post. Could save me some time if i had a starting point.