When it comes to making and editing photos I usually just write "A BA" and leave a blank where the backwards B would be and then type that B in by itself and flip it manually. However writing out backwards B on text, like in a blog post, I don't know of any way to type the Backwards B. Sorry. (Sorry I'm so bad at explaining things)
^^ There are different workarounds depending on where you want to do the reversing. I can show you how to do it on a web page, but the solution uses CSS, which may not work in a BBCode forum post. I'll sleep on it and get back to you.