This is a mock picture of how it will look
Awesome!
Unfortunately you can't change your name if you already have a Bnet account, so if you want a new name you will need a new account. Whether just having an obnoxious fake name is grounds for being banned from the forums is not yet known. I would recommend choosing a realistic name like I have, and keep the trolling subtle, if possible.
The process of having a longer name than allowable is below:
1) Download Firefox
2) Install the Firebug add-on for Firefox
3) Go to the Battle.net account creation page.
4) At the location where you enter your name, the normal limit is 15 characters for first name, and 20 characters for the second name. Doing this, you can have 32 possible characters for both your first and last name.
Right click the field where you input your first name and press "Inspect Element." You should now see a window pop up and input id="" should be highlighted. Find on that line where it says maxlength and click the number. Type in 32 and hit Enter. Now repeat this step for your last name.
You cannot do more than 32 characters for each field because they have server side checks.
Have fun!