So I've made a try finally!
I think your plugin is great but it lacks one setting: the database prefix!
My goal is to clone a wp site to another fresh wp installation not to just change domain name, but to create a clone site and then change only relevant contents and images, but mantaining all plugins and settings.
So, at 99% of times, I use a different database prefix.
I've tried a migration toward a new fresh 3.3 WP installation with different db prefix and your plugin correctly unserializes, changes and serializes again all data containing the old domain name, create backups and upload them to the new wp, but it uses the old db prefix and there's no way to instruct the plugin to use the new one.
Another thing I came across (but I made only one test) is that, even if I setted the correct path to the new wp site backup folder for your plugin, the backfiles was uploaded in the root folder, ie:
So I moved manually those files to the right folder and then I completed the migration process.
Let me know if you need more infos.