Unfortunately, we quickly found that the stock OS didn't support HDMI output so we've pushed out a beta firmware to address this in the beta testing scheme.
And yes, you're right. The solutions company only demo'd us with a specific HDMI cable that needs an app or apk to work with it. We can confirm this cable does work on Android OS but only with a beta firmware version (not stock). We're planning to merge the fixes made in the beta version into the stock via OTA update. An ETA haven't been decided yet but I presume it should be in the next month or so.