Top End Performance sells a cold-air induction device for Weber Downdraft carbs, listed on their website:
The page takes a couple minutes to load. Go to the bottom of the page, on the left, where it says "carburetor and air cleaner adapters".
It's basically what I want: something to enclose the top of the Weber and have one or two (they sell both) outlets for intake tubing that connects to a remote filter.
I called Steve at TEP and he said he doesn't recommend using them (even the dual-snorkel one) as he says it doesn't flow enough air. He said it should only be used for forced induction (blower/turbo) use only.
Somehow I just don't believe him, and will probably buy it one day, with both single and dual-snorkle top covers, and experiment with it myself. I hope it's not too good to be true...