I tried asking three questions of #WordPress.
Q. Can Core Contributors continue to log in if they're affiliated with WP Engine?
A. We can't answer that. Please don't ask.
Q. Do contributors need to sign a CLA?
A. No.
Q. Can GitHub users send a Pull Request if they're affiliate with WP Engine?
A. (After much wailing and gnashing of teeth) We don't know. No one from WordPress will tell the volunteers anything.
Very dispiriting.