If you manage to do so, please report.
No, it's not possible. It's a long discussion since years and I never saw any solution.As long as you can't modify the client's network protocol and how it handles the level management it's not a server issue.You also should think about why a WotLK client even should support levels above 255 if even cata and the patchs after never used the full range of 255 levels.And btw: Whatever you plan with you server ... the attractivity of a public server project doesn't lay in the amount of levels.
Fake video ?Hacking programs that modifies local client memory ?Using a modified client ?Arcemu never supported level above 255. Official they even didn't support level above 80. Like Trinity.But there is a simple way for you to test it.Have a look where the server is sending the network package. For testing purpose try to change this to let's say a fix level of '444'.I'm quite sure you will fail :-)If not, you can change the other stuff like db and core value lengths after that.
back when I started playing on private servers, I remember seeing people way over level 255.I believe the server I saw it on was based on arcemu.