My usual all year round 29" tyres are Race Kings. I don't tend to suffer too much in the gloop but I do have to concentrate at times ... anyway, there's a chance someone will be borrowing my bike for the BB 200 and I'd like to give them the best chance - I figure that if it's wet then the RK may not help their cause.

Has anyone any experience of the Maxxis Beaver? Thinking about something that stands a chance in the wet bits without too much drag on the other parts.

Got given some early prototypes of these by JustRidingAlong so used them quite a lot. They Are 1.9 size ones, not sure what the proper production sizes are, but these have been great. Super grip that encouraged me to search out off camber bits to play on.
You can get away with most tyres in mud on 29 wheels if you keep the torq down and lean the bike in more, but when you have some proper mud treads on you can really push it.
Bit noisy on roads but can't say they drag too much, did a few rolling race tests with mates and didn't show up any great drop in speed over nobby nics.
No experience of them on extended rocky areas though.

Bonty mud x's are pretty good in the gloopy stuff.

Cheers numplumz ... they sound like they could be just the thing. Production versions are 2" so not much bigger than your proto versions. I'm thinking the way forward might be one on the front and leave the rear alone.

Always found Mud X a bit on the draggy side, there's a couple hung up in the workshop but I'm loathed to fit them.