I think that Steve is correct: only for use with VPNs. I also don't understand the advantage of replacing the WebAdmin / User Portal cert - I'm not saying there isn't an advantage, just that I don't know of it.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005
Unless someone can tell us a better way to do it, I believe that a GPO is the best solution. This isn't the first time I've seen a request like yours here. You might look at Astaro Gateway Feature Requests and offer a suggestion if one does not already exist.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005