Still, the problem isn't so much the existence of a checkbox or two than a yet another case of stuff being hardcoded based on ship size and it being explained nowhere in the UI.
The problem is, that is dictated by FS2. In the main FS2 campaign, you scan the Iceni (as a whole ship) and the Sathanas (as subsystems). The cutoff threshold is defined as ship class size, and it just so happens that corvette classes and below use the scan-at-once behavior. The UI has nothing to do with this.
The only thing that FRED does is add a checkbox to switch this behavior. That's it. And that's the simplest way this problem can be solved, because if we had a series of four option buttons to properly represent all four combinations (capital-at-once, capital-subsystems, corvette-at-once, corvette-subsystems) it would get very confusing very quickly.
You should also consider that you are the only person who has found this concept difficult to understand in the seven years since the feature was added.
All that's really needed is a single checkbox to determine whether to scan the ship or subsystems
Which is what we have...
and possibly another checkbox to disable scanning altogether (so that you can make unscannable cargo)
That's easy. Just set the cargo to known and the cargo name to something like "Unable to Scan". Or even set the cargo to unknown. That's how it works in sm1-02 in FS2.
Well, how does one get to that page when they have a problem with setting up scanning-related stuff? It's on a wiki page for "minor features" linked to only from Portal:FRED, so to me it seems pretty... undiscoverable.
And this is a minor FRED feature. Tell me, just where on the wiki is there a page specifically devoted to "setting up scanning-related stuff"?
All I was asking about was A), allowing non-standard subsystem to be scannable, and B), allow scanned subsystems to display the cargo content when scanned (though admittedly I haven't tried this out on a transport class ship with subsystem scanning toggled on) using a checkbox next to the subsystem in the "initial status" menu to "display content when scanned" or something like that.
a) I think this can be done pretty easily. I've looked through the code and restricting the subsystems may simply be arbitrary.
b) This would require more work, although it can probably be tied to the existing "scannable" flag.