Without better knowledge of how the program works, any issues with cargo storage is an unknown. We can only guess what obstacles surround the storage of such data and the code that manages it.
My suggestion to the engineer upgrade commodities problem of cargo-blocking on ships, was that we be able to deliver cargo to Engineers in advance of using it.
Unless pre-delivery of cargo to Engineers is possible, upgrading ships with cargo-dependant mods is impossible on ships without cargo racks.
Module storage is another issue.
As long as we cannot store modules, module rebuy cannot have depreciation in value. Also, if we can't deliver to Engineers in advance, then the only alternative is takes modules as cargo and swapping from cargo to outfitting and back.
A solution to both could be two changes.
Store ships with cargo (this defines a maximum on what can be stored).
Transfer modules from outfitting to cargo and back.
Why?
You could buy a Class 5A Frame Shift Drive in a Cobra Mk III and "equip as cargo" (20T), take it along with the materials needed and the 1 x Magnetic Emitter Coil as cargo to an Engineer for the Level 5 FSD upgrade.
Return to your home station and "Transfer cargo to outfitting"
Swap to another ship, such as a Federal Assault Ship (which may have no cargo racks) and equip from outfitting as you do when you sell and rebuy currently.
Any of these options would resolve some of the current road-blocks and don't differ wildly from what we already have in the game.
Can it be done or not? Only the devs can say.
Unless frontier invented their own kind of computers (which i doubt), the practices of implementing game logic and storing game data are limited and predictable. (but i guess you can never know if human stupidity is at work and they wrote all the program code backwards or whatever)
Regarding your suggestions from a dev point of view:
- The difficulty of implementing storing ships with cargo is the same(if not slightly more complex) as implementing an actual storage.
- Swapping modules into cargo and back is pretty complex (and unnecessary).
Given they already have the ship storage implemented, doing a similar tab for storage of cargo and modules is just copy/pasting existing work with little modifications. Therefore I believe that the issue is not technical, but rather a marketing issue or content release scheduling issue. We don't need to think up creative workarounds or suggestions because the most intuitive and expected form of storage is already perfectly achievable.
While I agree that it's up to the devs on what can be done, to be honest the issue is not can they do it, but rather when they will do it (coupling it with some DLC like they did with Engineers perhaps).