I cannot think of one that you can do easily without customizing the registration form code.
Thinking outside the box, since the registration form allows you to set the role which the user will be assigned, how about you do a choice of region before you get to the registration form such as
Please select your Region
Then just direct them to the correct page that has the registration web part configured for that region. You could have multiple pages or just configure it on the fly via macro's and document name.
I do the same thing, by assigning roles based upon whether they register as a member, vendor, supplier, etc.