Replying to myself… 
I just ran into this response to “Replying to welcome message does not notify staff members (bug?)” from @codinghorror:
This will not be an issue in 1.8 as new user welcome messages will come from discobot, which will refer people to the /about page if they wish to contact staff. We’re strongly moving away from having the automated welcome be considered any kind of human to respond to.
So, I’m going to assume that, while it would be nice for a group to see responses at the moment, future versions of Discourse won’t send replies to the site contact.