Diff for merge conflicts in development branches

It would be great if Stash could do diffs/dry-run merges of multiple developer branches, and identify future merge conflicts before they happen. The pull request idea is great for figuring out merge conflicts with master, but what about merge conflicts with new work being done on other branches? They'll all get merged into master at some point.

In the following scenario, the VP of Engineering wants everyone to commit to master, in order to prevent merge conflicts down the line, so devs will know immediately if they're stepping on someone else's toes. But this means we're treating git just like subversion, trading 'svn update' for 'git pull --rebase'. If we can check for merge conflicts between existing non-master branches, we could give him what he wants and still maintain the functionality of Pull Requests.