Same here, but I can easily imagine the whole “responder” thing just means that if the session towards the peer is currently not established, an arrival of a payload packet that would normally trigger its establishing is ignored, but while a session exists, the “responder” setting plays no role.
Well, I would use a single mangle rule to force use of a “via-SSTP” routing table and use the on-up and on.down scripts of the ppp profile row common for all users to add & remove routes to that table, sourcing the remote addresses from the caller-id parameter (http://forum.mikrotik.com/t/ppp-on-up-on-down-variables/94303/1) and setting the gateway to the tunnel interface name.
Handling the case where the WAN IP of the “client” side is private is more complicated as the client has to learn its outermost public address first. In a country that takes the effort to actively prevent use of VPNs I don’t expect all the interested users to enjoy the luxury of a public address.