associations look as with the MongoDB backend now (global storage strategy)

Is it because you want to get the PR out before working on the next strategies? Just trying to understand the flow.

Regarding ids, as it seems in CouchDB one can't have composite ids in the sense of actual (sub-)documents which themselves have several attributes. The _id field thus contains basically a concatenation if the id field values. Note that the fields making up the id are stored as actual document properties as well. While this is a bit redundant I think it still makes sense for querying purposes. Not sure whether there is a better scheme for this, I'd be happy about any ideas.

I have to admit that you lost me. Could you give some examples of what's not possible in CouchDB and what alternative you have used?