Hi Bobcat,
I work for a company that does defense work. I'm currently working on a military satellite communications project. It's kind of difficult to get jobs in the sector especially if you're not a citizen.
When I saw my position advertised it never mentioned security clearance otherwise I would never have applied. I received a response and was told I had to be security cleared. Obviously as an American citizen, I did not have British security clearance. By chance I responded to their email saying that had I realized I would never have applied in the first placed but as you can tell by this response I rambled even further telling them that my husband is in the Royal Navy, etc. Amazingly enough I received a reponse from them requesting an interview. They said that since my husband was in the Forces that I would prove to show some loyalty to the country, etc, etc. To make a long story short, I was interviewed and offered the position. I've been cleared on the British side but they are still waiting for a response from the US. Until they receive a response from them I have to have an escort in certain areas of our site and if I go up and check on the Ground Stations and then there are certain areas I cannot go in at all.
My point is that it is possible to get work in this sector here but it's not easy. As with every job if you specialize in something your chances are better. Also if you're in the states and not even here yet and do not have any sort of visa then I would say your chances are very small at best.
I'm not sure if this was the sort of answer you were looking for. If you want to know anything else let me know.
Maureen