How to get output from helper for path argument in layout?

Try:

<block class="Magento\Framework\View\Element\Html\Link\Current" ifconfig="custommodule/general/enable_footer_link" name="custommodule-link">
    <arguments>
        <argument name="label" xsi:type="string">Custom Module</argument>
        <argument name="path" xsi:type="helper" helper="NS\CustomModule\Helper\Data::getFrontName"/>
    </arguments>
</block>

You can also pass parameters for the method like this:

<block class="Magento\Framework\View\Element\Html\Link\Current" ifconfig="custommodule/general/enable_footer_link" name="custommodule-link">
    <arguments>
        <argument name="label" xsi:type="string">Custom Module</argument>
        <argument name="path" xsi:type="helper" helper="NS\CustomModule\Helper\Data::getFrontName">
            <param name="name">value</param>
        </argument>
    </arguments>
</block>