Hirose make lots of small shielded connectors that should be fine for
1MHz. They aren't cheap. Unless you are operating in a particularly
noisy environment, though, I wonder why you couldn't just use a three
or four pin header (ground on just one side or in the middle, or
symmetrical ground on each side). Will you be connecting and
disconnecting them often, or is it more a set-and-forget application?

Have you done any phase noise testing on the LMS485? I'm wondering how
well they work for precision clock distribution. It's an interesting
idea. We find that ECL keeps close-in phase noise quite low, and could
drive the usual 100-ohm-differential RS485 bus, at least
singly-terminated.

Cheers,
Tom