Sorry to go back in time a little bit, but...
Rosie, have you tried
http://critters.org ? I haven't gotten started yet, but once I get my latest novel polished up, I was thinking of getting involved on there. They may be able to help you pinpoint at what point the big publishers might be losing interest in your story.
And if it turns out that you don't want to change the story based on their feedback, you could always hire a freelance editor to check things over one last time and then self-publish. It's not the "kiss of doom" people made it out to be a couple years back. You may not make a lot of money from it on the up-front, but it might be enough to get the ball rolling and have publishers take notice later on.
And you may find that the scifi reading community is more receptive to new ideas than the big publishing houses. Because, as you mentioned, I think they are being very conservative right now, not taking many chances on new ideas.
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