Apparently there once was an addon like that called Megaphone but it wasn't working anymore.
I made some changes to the addon bringing it up to date and also taking the opportunity to add a couple of new features like alternative fonts/styles and the ability to hide the leader's name.
DOWNLOAD from google drive.
You may also find it on Idrinth's Tools.
The only dependency is "LibSlash" but you probably already have it.
So here it is:

No special configuration needed. Just install the addon and join a Warband.
Type /mp to bring up the options:

You can chose from a variety of fonts and styles:

See the list with /mpshow, set the font with /mpfont (e.g. "/mpfont f8" or "/mpfont order").
Unfortunately these fonts are limited to the pre-existing system alerts.
You can also hide the WB leader's name with /mphide

Finally, note that in real RvR situations there are tons of messages appearing in the alert area. From quest progress or achievements to area names popping up on area change, these messages flood the notification area so you should consider removing some of them in order to have the important things be more prominent.
You can do that from the in-game options:

Hope that this will make PUG WBs more fun for leaders and members alike.
DOWNLOAD from google drive.
You may also find it on Idrinth's Tools.