Get all posts in RSS

I’m wondering how/if I can access more than the most recent X posts defined in the wordpress settings. I’ve seen plugins that migrate all blog content through RSS, haven’t poked around to see their methods.

Basically I manage a couple hundred WordPress blogs, and I’m building a newsletter generator for my clients. They want to be able to select a few posts and have the excerpts appear in the newsletter body. The sites are spread across multiple servers and the newsletter generator is being built on top of our CRM, so direct database queries would be difficult. RSS would be the cleanest, but I can’t seem to figure out how to access more than 10 at a time (when 10 is set in the admin).

Any ideas?

Codex has example snippet on how to use post_limits filter to override amount set in admin for feed.

if (isset ($query->query_vars['feed']) and ($query->query_vars['feed'] == 'ics')) 
    add_filter('post_limits','__return_empty_string'); 

http://codex.wordpress.org/Function_Reference/query_posts#Usage_Tips