As per my thought and logic it should work for your
As magento provide set and get method for different variable name so in quote also will work for you
just try like below
if the flag is set, making...
Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress(null);
OR
Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress->setCountry_id(null);
just try or find out in mage core to get your solution.