The answer is to add the following dependencies to my module.xml:
<?xml version="1.0" encoding="UTF-8"?>
<module xmlns="urn:jboss:module:1.0" name="at.mylib">
<resources>
<resource-root path="mylib.jar"/>
</resources>
<dependencies>
<system export="true">
<paths>
<path name="org/xml/sax"/>
<path name="org/xml/sax/helpers"/>
</paths>
</system>
</dependencies>
</module>