@Giles Jones

You described exactly what DARPA does. They put out a bunch of crazy specs and see if any commercial firm (or individual) can come up with a viable proposal. If someone can come up with a proposal they give you a sweet job and let you try and build it - doesn't matter if you make it work or not, you still get paid and get a good line on your resume. There are no non-bureaucratic "careers" at DARPA, you get hired to work on a project then your time is up when the project is over.

As an interesting footnote, often people who get hired by DARPA and successfully complete their project start companies immediately after their termination from DARPA to produce those projects for various govt. agencies. Pretty good work if you can get it. Check out their website sometime and look at the jobs section...