Technically, the only difference between form view and list view is that in list view, records are shown one beneath the other - so you can scroll down/up to the next/previous record. Thus, if you make your body part tall enough, you can place a portal there - regardless of the layout being viewed in list mode.
Conceptually, I think users are more accustomed to drilling down the hierarchy, e.g.:
1. List of Clients:
Select a client / Add a new client
2. Selected Client in Form View:
Edit a client / Select a return (from a portal) or add a new one
3. Selected Return in Form View:
Edit a return / Select a document (from a portal) or add a new one
That's exactly the way I initially envisioned it working but I didn't know if there was a better way. I'll give it a shot using that approach and let you know how it goes.