You have to use a variation of that encoding like base64url: http://fr.wikipedia.org/wiki/Base64
I don't think ASP.NET directly support that encoding. You can write your own functions or for do that encoding and then encode for URL.
Anyway to do it cleanly in MVC, you will probably have to override a bunch of stuff.
Are you doing all the encoding by hand or have you expanded MVC to support that?