In a forum post, a user reported that he cannot use the web boot strapper to install since the downloads are blocked by their corporate fire wall.
You can use the the "big" redistributable package will allow you to install NETFX without downloading anything. There is another trick you can use once if you can get the You can run the full bootstrapper with the /x command line option which will extract all of the files to a local or network location. From there you can install the .NET Framework 3.5 and will install from that local layout.
.NET Framework 3.5 full install package (x86 and x64) - http://download.microsoft.com/download/6/0/f/60fc5854-3cb8-4892-b6db-bd4f42510f28/dotnetfx35.exe
Let me know if you try this and if it works for you.