In reply to Motown:
Oh I'm so so so so jealous!
Haddash would definitely be a struggle in a normal car. You can camp underneath the crag. You have to walk through the village to get there, so long trousers and long sleeves are a good idea until you've got round the corner to the crag. The local pomegranate-growing man will then come and tell you all about his water channels.
Wadi Maih (or Mayh) is easy to get to from Muscat, especially now there's the road over the white mountain. Take your trad rack if you're going there. You can camp in the wadi where it widens out.
Have fun!