Just setup htaccess on your original server to redirect to the new URLs. If you care so much about google juice, then you can spend a day doing this, instead of requiring posterous to solve the problem for you.
DNS will resolve to posterous' servers. It's out of your old server's control by that point.
Unless you're talking about different domains, e.g. moving from
mycoolblog.com self hosted -> myawesomeblog.com on posterous
in that case you could do what you said, but I don't think that's what the OP is talking about. the issue is when you use the same domain on both.