I have successfully built Cocoon 2.1.10 with eXist as a block and run it under tomcat on my local machine. I have deployed it as a "complete" cocoon (all blocks with samples) and as a stripped down version (with select blocks and associated samples as well as select blocks with no samples).
What I've discovered is that the online instructions for building eXist as a cocoon block do not tell you that you need to run the eXist build first, before injecting it in to a cocoon source tree (we need to tell Wolfgang to change the directions online).
What I haven't discovered is how to get in to the eXist db inside of the Cocoon with no samples. Trying the obvious options do not work. I need to track this down so we can have a nailed down Cocoon to deploy without all of the potential security holes offered by the haphazard sample apps that come with the basic build of Cocoon.