I believe it's possible for plugins that properly use GoCD's logging support (i.e they direct their logs to the GoCD plugin subsystem, rather than using their own logging configuration) by defining configuration in your logback-include.xml like the below.
<logger name="plugin.cd.go.authentication.passwordfile" level="INFO">
<appender-ref ref="MyLogstashAppender}"/>
</logger>
From what I can tell, you'd need to declare one
<logger> override per plugin unfortunately, with
name="plugin.${pluginId}". The
${pluginId} will be the same as you see in the suffixes for the existing plugin file logs you should have sitting alongside go-server.log.