Such workaround should work, in case if you have already implemented @EmbeddedId or @IdClass in OGM

Yes, these are supported already.

May I also state my opinion about support of sharded collections?

Absolutely, any feedback is more than welcome!

Without sharded collections MongoDB and others such as Cassandra - almost useless.

I wouldn't exactly say that these stores are useless without sharded collections, e.g. MongoDB adds much value by its support for nested/structured data, no need for having a physical schema in the datastore etc. But of course I understand that sharding is an important feature for many, and we will surely add proper support for it. Really it's just a matter of resources and prioritizing. But the good thing is, it's all open source, so you can push for a certain feature just by sending a pull request