Is there some Plone add-on that would add the concept of "assignees" tied to workflow transitions?
There is some customer request to submit a document for review to a particular group of reviewers or a particular reviewer. Clearly, this can be implemented with additional fields (e.g. through a behavior)..perhaps there is something already existing?!
There is the already very long existing concept of registering python script that run on a workflow transition. There you could basically do anything you want to 'tie' a user to the content item.
It's more the question how tight you want security controlled that will make this more complex. The default. Private/submitted-for-publication/published workflow depends on the reviewr role being able to move all content in submitted state to published.
the second question if listing those 'to be reviewed' items is a necessity. There is a portlet and I think view for this now, but that would list all items with the state, no feature for subgroups of reviewers yet.
Hnm. Thinking/writing out loud... this can be done easier: With the Sharing page you can add additional local 'reviewers' to a section/content item of your site. If the listiing is not a requiremenbt you could create a content rule that mails the localrole reviewers that an item in that section has been submitted for publication.
And that's not True either: the review listing will take into account the persmissions that the role grants you on the item, so the listing will only show that users review items. Then you're actually done.
Unless I totally misunderstand you and you are talking about 'taking a look at the document' in general as 'review' and don't actually mean that the Plone reviewer then is allowed to approve and publish the document.....
I'm on your side. The problem is as usual: reading between the lines of customer requirements and having an idea how to implementation theses wishes - also with regards to the budget 
Wasn’t there something similar available in Plone Intranet?