I think the brief answer is yes, the answer seems to be well-know to the grand master of billiards, Bunimovich himsef. See http://www.scholarpedia.org/article/Dynamical_billiards and
the references quoted therein.
The above example of rectangular billiard with round corners has several ergodic components in the invariant measure so it is not uniquely ergodic. But as long as there are round corners (focussing boundary components) there will be a continuous ergodic (w.r.t Lebesgue) component, while others, corresponding to periodic orbit (families), will be singular.
Other examples of this type of mixed invariant measure include the "mushroom" displayed by Bunimovich as Figure 3 in the above article - they are thus neither fully chaotic nor fully regular (the sharp rectangle would be non-hyperbolic). A billiard in such mushroom has one integrable island formed by the trajectories which never leave the cap and it is chaotic and ergodic on its complement. A mushroom becomes a semi-stadium when the width of the feet equals the width of the hat. Combining mushrooms together one gets examples of billiards with an arbitrary (finite or infinite) number of islands coexisting with an arbitrary (finite or infinite) number of chaotic components (see ebit)