I've used castable for complex shapes, but not an entire oven. I think a properly done cast dome will perform every bit as well as a brick dome, but doing it right isn't cheap, and I get the feeling you are after bang for your buck. To properly cast an oven you are going to need a quality multi part form(time and money), a vibration table or vibration apparatus of some sort(time + money), a quality commercial castable refractory(money++), and stainless needles(money++).
In your situation I would probably build a quality base(like they describe in the Pompeii plans), insulate it well with board or vermicrete, put a firebrick floor on it, and then build a cob oven. It is easy and cheap, but functional. If the bug bites you and you think it will be worth your time and money to built a true masonry oven you simple demolish the cob dome and build a masonry dome on top of the stand, insulation, and oven floor you already have.