I've actually watched HBO Go on my big screen using the MHL on my Evo 3D. Is it being blocked a new development or something?
I had pretty much the same situation as the original poster. I have Directv, something was on the HBO Go app that wasn't on Directv On Demand, so I connected my phone to my tv and watched it that way. This was several months ago though so it could be blocked now. I haven't come across anything on that app that wasn't available on Directv On Demand since then. I actually read shortly after that Directv was supposedly going to have all the same content available On Demand that was available in the HBO Go app in the near future, so that may have something to do with it.
For what it's worth, the picture quality was pretty crappy. It was still watchable though.
That would be pretty disappointing if they were blocking this. You pay for HBO, who cares where or how you watch it? I get that maybe someone will let their friend use their Directv login or whatever. But don't they limit the number of devices that can view the content anyway?