How to insert events and attributes into wp_* links

This seems so simple yet I cannot achieve it without modifying the core code.

I want to be able to insert either an onClick event or an attribute like link=”attribute” into the output html that derives from things like:

<?php wp_list_categories(‘show_count=1; ?>
or
<?php wp_list_pages ?>

so that the output would be
Uncategorized
The problem is how to inject rel=”MyValue” onclick=”MyValue”
into
<?php wp_list_categories ?>

Can anyone help?

Many thanks.