Can I put a filter in resin.conf that get's executed before the filters in web.xml?

All the web.xml definitions are before the <web-app> in the resin.conf.

Here's some more details:

All the <web-app-default> are concatenated in order. The <web-app> in the resin.conf comes at the end.

In the standard resin.conf, the first <web-app-default> comes from the resin:import of the app-default.xml. The app-default.xml has a resin:import of the web.xml.

In other words, any <filter> in the web.xml appears before a <filter> in the <web-app> in the resin.conf.

You could add a <web-app-default> before the <resin:import> of app-default.xml to add a filter before the web.xml.