It turns out it's pretty simple.

In the autoposting options for your Posterous blog, add your Twitter account and setup the advanced formatting to include {{subject}} {{short_post_url}} {{hash_tags}} which will keep the url short. If there is space it'll include your posterous tags as #hashtags in your tweet. So then you just gotta keep your titles shorter than about 120 characters (144 minus 1 short url). Then as long as you entered your twitter username in your yammer settings, just include the tag "yam" in your post, and it'll appear there.