How about looking at the business case for a viewing area, BAA seem to be primarily a shopping mall owner with a few planes around now anyway, so I would have they would love to rent a bit of space somewhere on their airports to a company providing facilities. Thats all they do with shops anyway, rent out space.
If a business came to them with a plan to rent a bit of ground/terminal etc. that they could not rent out at the moment, and addressed issues such as security, I'm sure they would jump at it. BAA don't really want to run shops, so they lease out the space, they don't really want to run restaurents so they lease the space for them to another company, why should their thinking on facilities for spotters be any different?
If airlines can get away with piping the comms feed to each seat on the aircraft , this new business might be able to get an exemption to allow piped feeds from ATC/Ground/Tower to headphone sockets on the viewing are.
Just a thought