I actually have a friend who is finishing up his PhD on Nabatean studies here in Liverpool.

As far as I can tell from previous conversations with him, Nabatea was an independent kingdom from the Seleucids, and did not fall under anyone's control until Trajan's time.

Therefore, it's an independent kingdom, as far as I can tell.

However, MAS, you're probably going to find that, historically speaking, the Nabateans like remaining neutral for the most part. If you can give them some sort of economically sound reason to join your cause, then they might get on your side.

Of course, that's up to Caius ;).