Not me. Not how I use my Display Case
Shelves are too fiddly so I gave up using them. I am a collector so I am more interested in what
https://museum.loathers.net/ says than what
https://pricegun.loathers.net/ says. To the extent that I choose to play The Mall Game I use some command line scripts, rather than relay scripts or GUIs, and those have options to include or ignore Hagnk's, the closet and/or the display case.
if you wanted to add it to mafia I probably would not oppose it but I would like it to be opt-in so I never know it is there.
As an alternative, there are relay scripts out there that do similar things and I might wonder about making one of them use the display case as a source.
As a collector I am used to a lot of scripts that inspect all items. My computer is fast enough. I can imaging a new comprehensive Item manager that displays everything the Item manager does, displays everything you want, and enables appropriate commands. It would include a column for where something is - inventory, closet, Hagnk's, display case or "not owned". Commands would be enabled or disabled as they made sense. You could add a column for display case shelf which would be reported if in the DC or use when putting an item in. It would probably need a data file for support
As I write this I feel like I am describing an inventory management script such as Bale's Inventory Control or the renamed fork philter(?). So maybe a feature request or fork would also get you what you want.