I thought for some time on how you could do this easily, but didn't find an easy solution yet for you, sorry.
CB is moving away from PHP-controlled templates to database+CSS-controlled templating using an internal html generation engine, as this offers many benefits.
For backwards-compatibility reasons with the 100+ existing CB plugins we didn't want to change the registration page controls and html markup yet.
I see 2 solutions, both not easy and not clean:
1) modify the html generator in plugin.class.php cbTabs class to your need (that one is now used for all fields displays), but it's a no easy change.
2) use a javascript script which changes the html of the page after page load.
CB 2.0 will allow that and much more, but it's not available today (and not next month eiter
).
Sorry, out of CB 1.2 out-of-the-box scope.