On a hardware level the 360 shouldn't be able to output DD+. The 360's HDMI audio support is exactly Toslink mapped to HDMI. Whatever you can get out over optical is what the HDMI port will also do.
If HBOGO supports DD+, then it would need to be decoded on the 360, then recompressed to the 360's 640kbps DD stream. This isn't quite as silly as it sounds, as it could result in what sounds like a 640kbps stream, using 384kbps of bandwidth, recompressed for output that ultimately sounds like a 448kbps stream. Really though, does it matter? Point to a person that thinks they can tell the difference between DD+ and DD at the same bitrate and I'll point to a person that is delusional about their human audio subsystem. I would also not be surprised if DD+ was being used simply as a cheaper bandwidth solution. 256k DD+ is roughly the same quality as 384k DD, which both sound the same transcoded to 640k for output.
I can't personally verify that this is exactly what the 360 is doing. I would like to pay HBO $15 a month, but they won't let me give them my cash. They want me to go through this so-called "cable" thing like it's 1994.