My_Custom_Trait_Extension_B_Model_Product

gistfile1.php

trait My_Custom_Trait_Extension_A_Model_Product{
    /**
     * This method loads the original, rewritten class of 
     * Extension_B_Model_Product. Mind the shorthand - you need
     * to load correct original class.
     * $this becomes later part of My_Custom_Model_Product context
     */
    protected function getInstance()
    {
        if ( is_null($this->_instance) )
        {
            $this->_instance = Mage::getModel('extensionb/product')->load($this->getId());
        }
        return $this->_instance;
    }
    /**
     * Implement original class' isAvailable 
     */
    public function isAvailable(){
        return $this->getInstance()->isAvailable();
    }
    /**
     * Implement original class' fixSomethingByProduct
     */
    public function fixSomethingByProduct(){
        return $this->getInstance()->fixSomethingByProduct();
    }
}