<checkout_cart_index>
  <reference name="checkout.cart.methods.onepage.bottom">
     <block type="checkout/onepage_link" name="re.checkout.cart.methods.onepage.bottom" template="custommodule/checkout/onepage/link.phtml"/>
    <!--  ALSO TRIED BELOW

     <block type="checkout/onepage_link" name="re.checkout.cart.coupon">
        <action method="setTemplate">
           <template>custommodule/checkout/onepage/link.phtml</template>
        </action>
    </block>  -->

</reference>
</checkout_cart_index>