You can now use the YouTube tag is Visitor Messages, along with the other custom BB tags we have added over the years.

For example(remove the spaces):
[ youtube=cool video ]A67ZkAd1wmI&feature=related[ /youtube ]

between the tags you put everything after http://www.youtube.com/watch?v=



HTML is not allowed anywhere on TFF due to the high risk of XSS attacks from abuse of code. I am unsure why it was re-enabled on certain limited cases, but the security hole has been closed. I am sorry for any inconvenience this may cause you, but it was never allowed in the first place.

If you have any questions on this matter, feel free to ask. Thanks.