If you dare, that is.
I would like to know why is your blog “under” MT/WP/XYZ/etc ? I think 99% of people are using (stuck with) one of the popular blogging platforms as a price they are paying for decision made about this either long time ago, or some time ago when they have not had sufficient knowledge/experience about the issue.
In reality, it is notoriosuly difficult to switch blogging platform. This is partialy because “big names” do not have to be helpfull in providing this functionality. Here I mean first and foremost: MT and WP.
Ok, with my architect hat on, I can think of “blog engine markup language” or simillar planetary effort (I even think such a project existed or exist?). But with my blogger hat on I want a quick fix. That works too.
Maybe the clever team behind Windows Live Writer (WLW), can add the functionality of taking ALL of the material from Blog A, and publish it to blog B? Even right now this is doable with the current release. Although one has to do it manually, and thus makes it hardly pratical.