Related-file entry using portals

I'm not sure if this falls under portals or relationships, so I'll see what you folks have to say. Here's my situation ...

The relevant files in my database are structured as follows:

--2 files, "companies" and "products", in a M2M relationship

--1 join file for the M2M, "line_items"

--"products" file is a hierarchical listing (hierarchy done through self-joins)

What I want is a nice way of entering, and showing already-entered, data into the "line_items" file FROM the "companies" file. The entry choices (in the form of a hierarchical list) are individual records in the "products" file.

My preliminary idea was as follows:

--The entry is performed using a portal.

--The portal is based on a relationship between a (drop-down selected)global field in the "companies" file, and a calculated "category" field in the "products" file .

--The portal then shows the products from the specified category.

--In the portal, next to the product names, would be a calculation graphic (a checkmark), indicating that the company does/does not have that product. (Note: the checkmarks should correctly show related items from "line_items" as the user scrolls though records in the "companies" file - sorry, no scripted calculations allowed)

--The checkmark graphic in the portal doubles as a button to set/clear that particular product (ie: to make or delete that particular record in the "line_items" file.

The major roadblocks I've come up against:

--How to have a "global, calculated" field in the "products" file which always contains the ID of the record currently being browsed.

--How to calculate a "product_exists" (0,1) field in the "products" file to indicate that the current company makes a given product. (ie: a real-time find related records calculation)

:

The reason I'm not simply using a checkbox-based value list is that I need the M2M structure for reporting purposes, and need the entry options structured as a hierarchical list.

All of this seems a little convoluted for such a simple task, and I'm wondering if there's some built-in feature that would help with this, or some function I just don't know about.

Any help on the roadblocks above, or offering a completely different perspective on a solution to this problem, would be greatly appreciated.

Thanks!