I’d like to perform some string manipulation on the body of the post as each item in all of my feeds is generated. (For example, globally replace all the <blockquote tags with some <blockquote style="…, and to add linked taxonomy terms.)

I don’t want to have to create custom templates for all the different feed types—if I’m understanding correctly, that would be one way to do it.

There’s a hook for the_content_feed where I can manipulate the content. But by that point, calls like get_the_tags() no longer seem to work.