I'm against closed studbooks overall, regardless of breed. Whether the breed is currently suffering or not, having a closed studbook is inevitably detrimental in the long run.
I don't know enough about the breed OR breeding to claim I know what the best outcross options would be, so take the following with a grain of salt.
My first thought would be field-type English Setters/Llewellin Setters to help retain hunting instinct. They are somewhat similar in coat/build, and not too different in temperament that it would be hard to breed back to true Brittany temperament. Llewellins, from what I know (fairly cursory research) are fairly healthy dogs. Hip dysplasia and hypothyroid are the two main things it seems, and those are already present in Brittanys.
The pointer is another option. The hunting style is similar. I am not too familiar with temperament and health issues.
Maybe the GSP - again, similar hunting style, somewhat different temperament though. They have some health issues not common in Brittanys, so that's something to consider. Cancer is more prominent in GSPs than in Brittanys. DM is something else I'd be concerned about, because from what I've seen it's not really an issue in Brittanys but is known in GSPs.
Vizslas... my experience with Vs has been that temperament-wise, they're very similar to Brittanys, and from reading they're similar in style as well. Again, I'd be concerned about cancer. Epilepsy is something to look out for in Vizslas, but it's also something to look for in Brittanys, so it wouldn't necessarily be something new introduced to the breed.
I don't honestly have any goals for outcrossing, so to speak. Genetic diversity is enough for me, and Brittanys aren't so plagued by any particular health issue that I think outcrossing alone is the solution, such as the Dalmatian project. I'm not sure about approval, honestly, but I like the Chinook club's process on outcrossing and breeding back to "purebred" status:
http://www.chinook.org/faq-cross.html