Setting a default value for a config defined value?

You can easily add default values for all configuration settings in your config.xml:

<config>
    <default>
        <cms>
            <wysiwyg>
                <enabled>0</enabled>
            </wysiwyg>
        </cms>
    </default>
</config>

At the moment I'm unable to map your system.xml setting name to the node in the config.xml

I would say it is: catalog/my_val/my_inner_val, means:

<config>
    <default>
        <catalog>
            <my_val>
                <my_inner_val>1</my_inner_val>
            </my_val>
        </catalog>
    </default>
</config>

Small tip in the end: Be careful with the int settings. Sometimes magento uses 1 and 2 for yes/no, on/off, etc.

You can assign default values from your module's config.xml. Define there node with such xpath

config/default/catalog/my_val/my_inner_val.

Hope this will help.