I'm trying to figure out if I'm going to need any Whatchamacollars for
my Big Dummy. Are the racks normally just slipped inside the frame's
tubing, and the Watchamacollar clamps that junction? Does that mean I
would need 4 of them to make the rear rack solid and not let rain and salty road-spray in?

Is there something else that would work? $35 for what looks like a pair of seatpost clamps is pretty pricey.

Need them? No. In addition to weather-proofing they provide extra rigidity. Mine came with my Big Dummy (used).

You could use a piece of inner tube for the weather-proofing. That's nearly free.

I believe Xtracycle states that they are necessary if you are putting a child seat on the Flightdeck. I like them as they do keep things secure and rigid. As for weatherproofing, they claim to keep water out so that is good but it's not like I'm checking to see if any water got in after a ride in the rain. Yes 4 is ideal and they are overpriced.

I went ahead and ordered two pairs. I'm putting a child seat on it and I want it solid. Too busy with business and family to screw around trying to rig something.

I use two compression straps. Same effect, though I'd get the collars if I had kids.

+ 1 - slip some cut up sections of road bike inner tube over the v-rack/frame juncture to waterproof them.