Sample code for Hierarchical Select in a block as a taxonomy filter

Of course this is possible. Just write the necessary Forms API code. There's plenty documentation on that. Look at the top of API.txt to see how to write a HS form item.

Interested if anyone found some more info about using the FORMS API with HS. I read the docs in HS module and DO and still feel a bit confused.

I would also need such a block

While it's not up to me to educate users about Forms API, I'll provide a piece of sample code, because it's so often requested :)

Done: http://drupal.org/node/534986. And commented on the forum post MattDrupalee started: http://drupal.org/node/433672#comment-1867690.