Can WP User Groups support grouped users permitted to post on behalf of other grouped users?

[robertandrews]

Could WP User Groups (or any other tools) help enable the following user/permissions model, designed to support agency staff posting on behalf of client executives?

Grouping:
Users should belong to one of two classes of Company: either Agency or Client.
Clients should relate up to an Agency. An Agency may have more than one Client on its books. Let's hope it does.

Permissioning:
Any User of a particular Agency should be able to post, edit etc on behalf of (from the account of) a User belonging to a Client of his/her Agency.
Posts should be hidden from both Agency Users and Client Users if those posts are not by a User in the same Client group as the logged-in User, or not by a User of a Client which is related to the logged-in User's Agency.

Example:
Any User in Wonder Agency can write blog posts on behalf of Users in Serious Client, like Bob Exec and Barbara Exec.
Users in the other Agencies cannot see/edit these posts in the backend.
Wonder Agency may have other Clients for whom they can do this.