[SalesForce] Lookup Field on Custom Object related to Activity (Task) (aka create a related list on Activities/Tasks)

This question seems to have been also asked here: Tasks and events custom related list with no resolution.

I have the ZenKraft Shipmate UPS package installed in my organization, and I'm looking to relate the UPS Shipments from it to an Activity Record Type (specifically Task, not Event) we have called "Warehouse Order."

To outline use case: reps here create Warehouse Order tasks and assign them to our warehouse guy, and he creates the shipments and closes out the tasks. I need that shipment to trace back to the warehouse order that created it for reporting purposes.

I obviously tried going into the UPS Shipment custom object in the package and creating a lookup field to Task….and hit the same wall as the gentleman above, as I can select any other object in my org aside from Activity objects on the screen when it asks what the lookup field should relate to (not to be mistaken with the "Related To" field on Tasks). I need a related list on Tasks that lists these Shipments (though it is a 1:1 relationship, not 1:n) in part because I need the "Related To" field on the Warehouse Order Task for something else.

I also cannot make UPS Shipments a detail in a master-detail relationship with the Warehouse Order Tasks for two reasons: 1. we occasionally have a shipment that comes through without a warehouse order and (more importantly) 2. it's impossible to add a master-detail relationship to a managed custom object.

Does anyone have any idea how I could achieve this? I am not particularly sure how to write Visualforce code (I've tinkered, but am very much a beginner), so I don't really understand the solution proposed in the above thread. Regardless, unless I'm misunderstanding it, I don't believe it would solve my problem the way I'm looking for anyway.

Also, fwiw, my org has been using warehouse order tasks for shipping since long before my time — aka going on 3 years now — so it's not quite as easy as ditching the Warehouse Order task in favor of a custom Warehouse Order object. Of course, if that's the only way and anyone has tips for migrating past data and switching over the workflows, I'd be glad to hear them.

Thoughts? Thanks in advance if you can help at all!

EDIT to add point brought up in comments

There's a chance I'm limiting my thoughts in how this could be solved. Perhaps Visualforce? So as a tl;dr list of raw basics, I only really need:

1. to launch the "Wizard" VF page from the package,
2. link the UPS Shipment created in the Wizard to the warehouse order, and
3. pull a ton of fields from the Warehouse Order's Related To record into the Wizard for the shipment (this is reason #2 that I can't change use of the W.O's "related to" field. Reason #1 is that Warehouse Orders are created before the UPS shipments without exception).

Edit 2: Screenshots
So I'm following the steps outlined here, except Task (and Event) isn't an option in the dropdown when creating a lookup (not screnshotting that, but trust me, it's not there), and if I try to create a custom address source without creating the lookup first I get this:

Which means, as you can see, if I create a new Shipment, I can't have it autofill from the warehouse order, which would be pretty clutch, but I also have no way of attaching it to the warehouse order either.