It's a nice thought however I don't think this will work practically. You shouldn't be forced to thank or post just to view content. 90% of the content on this site will be found somewhere else on the web anyway.
This idea, I think, will encourage spammers due to the fact that people will post even more stupid posts just to be able to view content.
So from what I understand you want people to post about the topic before they have even seen the content. Now I am not the world's most intelligent person but surely if I was going to comment on what the topic was about, I'd have to see it first, surely?
people would just post 202020202020202020202020
so it would unlock!?
Then they would get a warning from the staff?...
If you are going to bother doing this, as you say it should stop leechers however it would encourage more spammers. Let's face it at the end of the day, you'd rather have pathetic leechers than idiots spamming even more all over the forum.
It's not the best of idea but that's not why i'm disagreeing. I'm disagreeing because i'm 80% sure that there is no [lock] and [/lock] tags in BB Code which is what the youtube img and url= is. So it would not work out since there is not a code for it and I'm also sure you can not just make one up.