I think it would make all kinds of sense if you could send one (or maybe two) companions that you have romanced to Home Plate.

Writing the code/scripts to have the settlement functionality work with interior cells while also remaining connected to the settlement network on the exterior cells might be a bit much for the engine. Obviously, scripts are running in real-time, as you'll occasionally get messages about a settlement needing whatever resource, but when you get to that settlement, the problem is already resolved. The overland map is still loaded even when its graphics are unloaded. It's always running its scripts. Interior cells completely unload when you move through a new load door. To have all of that script functionality remain active in a second instance for an unloaded interior cell would be technically possible, but would introduce a whole range of things to break. (Not the least of which is interfering with the existing exterior cell settlement scripts, which already appear to be quite buggy.)

To put rainbow colored sprinkles on this birthday cake of horror -- Diamond City itself is an interior cell. Which means that Home Plate is an interior cell inside of an interior cell. In order to have the Workbench in Home Plate linked to the market square in Diamond City itself then those market stalls linked to the exterior cell settlement system, you're dealing with two secondary sets of settlement scripts running for two additional locations -- both of which will be 100% unloaded by the engine when you're not physically present in the cell.

The present engine can't seem to keep reliable track of a single settler you send from Sanctuary to Red Rocket. Can you imagine what would happen if that script tried to send a settler from Sanctuary...through the wasteland...through a load door...through the unloaded interior cell...through another door to another unloaded interior...to interact with an unloaded workbench...and then link its goods with an overland workbench...?

The next time you entered Diamond City, the game would inform you that all of your workbenches are empty and Dogmeat somehow fathered Strong's child.

The home in Diamond city is crap.. really..

Well damn!

When you put it that way it does sound like horror instead of "neato."

I don't know enough about it to know for sure, but I fear you are correct . . . still, they've got it doing some pretty impressive things that it didn't ever seem to do before: For example, as you note, it appears that the overland grid does run in real time and actors movement on it is processed in real time.

Take Dogmeat to Diamond City. Get Piper as a companion, which will send Dogmeat home . . . actually hold on . . . not _positive_ how it works if you do this from an interior cell, but I'm confident it works if you were to acquire a settler in an exterior cell: dogmeat would start walking for the settlment you sent him to. He will literally walk all the way there it seems. If you fast travel to a point that is assuredly on the route, you can catch him as he leisurely strolls along heading for his new home.

Assuming it does also work if you change companions inside D.C. then it might even be possible to use that same script as a template to link the interior work bench with the other ones. Since no companion can be sent to Home Plate, I think you'd have to use console to do this but the way to test it would be to go to Home Plate with a companion (say Piper in this instance) and then console in another companion (say Dogmeat), then switch and send Piper to some other settlement (say Oberland or Hangman's). If her AI can take that input from the user and effectively navigate through the door from Home Plate to D.C. Market, then out the door of D.C. into the Fens, and make her way to another settlement, then I would think it should be possible to usewhat ever scripts are making that possible and model something very similar to make Home Plate link up to the other settlements too.

Would it be possible to cheat the system by placing home plate in the exterior world somewhere inaccessible. Like under the map, or outside its boundries? Inside a mountain? Or even just in the outside map location for diamond city that probably has nothing but an empty placeholder? Then have the load door transport you to that spot on the exterior map, then when leaving transport you back to the interior cell of Diamond city. I mean you couldn't link diamond city, but you could link homeplate. Although, some people brought the main cities in skyrim out to the main map, so you could do that as well.

Drum roll ..

The 1.3 beta allows workbenches at Home Plate.