@quomist - Yes, you can purchase HydraTubes independent of each other.
@lepstadder - You won't have to worry about having to vape your bowl really quickly due to the vertical mount. It would be similar to anyone that has used an Extreme or Zephyr. I wouldn't advise loading a bowl and leaving it there over night but if you are playing a video game or watching a movie, you can finish your bowl at your own pace.
@Alacc81 - PPHT (Project Pillar HydraTube) will commence shortly
Here are some Sovereignty pics from my own collection