>>> Mark's message says that all messages to your "+owner" address from Russian (.ru) addresses will now be blocked.
Not quite; Mark's message said from non-members, not from everyone from that domain.
>>> If those replies during the joining process are blocked they will never be able to join!
I could be wrong on this, but it seems the moment one clicks on "Join" the system treats them as "members" right-away albeit not yet approved (provided of course "Restricted Membership" is ON). This is why even if they haven't been approved yet, they can still set their group subscription message delivery options, I mean they are landed in that group subscription page after all, to me that means they are "members" but not yet fully capable (i.e. approved) of "accessing" all group functionality. So it seems to me their messages wouldn't be subject to this new restriction.
I guess Mark would have to clarify what happens in this special case.