How can I register a secondary servlet with Spring Boot?

Just add a bean for the servlet. It'll get mapped to /{beanName}/.

@Bean
public Servlet foo() {
    return new FooServlet();
}

Also available is the ServletRegistrationBean

@Bean
public ServletRegistrationBean servletRegistrationBean(){
    return new ServletRegistrationBean(new FooServlet(),"/someOtherUrl/*");
}

Which ended up being the path I took.