There's no way to specify a different filename for logback-access.xml in LogbackValve because its setter method is missing.
The following snippet of server.xml should work:
<Valve className="ch.qos.logback.access.tomcat.LogbackValve"
filename="my_logback_access.xml"/>

But the default logfile ${catalina.home}/conf/logback-access.xml is used instead.

Its counterpart for Jetty, namely RequestLogImpl provides such a method - so please add it for Tomcat too.