System volume locked on Mac mini when using HDMI audio
The reason is that since you use a digital link (the same behaviour is exhibited when you use the optic interface) the modulation must a high as possible to get the best sound quality possible.

This is because when lowering the modulation you reduce the amount of significant bits that "describes" the sound : the resolution and dynamic range of the source signal is lowered. 

So, even if it is theoretically possible, that's not a good advice from an audiophile point of view. I guess that's why Apple decided to lock the volume control when using those links. 

Regarding the workaround, you may try to install soundflower : http://hints.macworld.com/article.php?story=20090111110741237