Hi Andrew,
I have the same concerns as Phil above. In your example, with the accessallgroups capability turned off (prevented) but the canposttomygroups capability turned on (allowed), a teacher can post to all the groups that (s)he belongs to but cannot view other groups at all when the forum is set to separate groups. With a large number of groups, separate groups is the least confusing option for students, I think, but there are legitimate reasons why a teacher would like to have access to ALL groups (observation, checking the general progress of all groups in a course, including one's own, etc) but also have the option to post only to those groups to which (s)he belongs.
And yet as functionality now stands (Moodle 3.1), with the accessallgroups turned on, the teacher is allowed to see all separate groups, but any post made using the Post a copy to all groups option sends that post to ALL groups, including the groups that the teacher only wishes to observe. However, failing to use that option, a teacher who wishes to post to all her/her groups at once is restricted to choosing ONLY ONE of the groups to which (s)he belongs and, hence, has to repeat the posting for as many times as there are groups belonged to. This is counter-productive.
To use your example above, although I belong only to groups A and C, I might have sound reasons for wanting to view groups B and D while keeping them as separate groups for students to avoid confusion. At the same time, I want to make occasional posts to my groups A and C in a single process, i.e. without having to make the same post first to A and then to C. At other times, I may wish to post only to group A.
I think Phil's suggested solution to this problem is both elegant and intuitive. I would certainly vote for this solution if it were posted to the tracker.