Hi,
Thanks to plugins and view override, you can already do a lot of stuff.
With a view override, you can change the way that the vendor order is display and add a button to edit an order product (in a popup, it's better).
Instead of using the default "order" controller of HikaMarket ; you can create your own custom controller thanks to a custom plugin.
The plugin "mangopay" use that system to create new interfaces in the front-end to let the vendors manage their mangopay account.
So ; thanks to that custom plugin, you can only display the edition of the the item custom field (and also have the code to handle the content and modify the products in the database).
It is not easy in the way that it requires some code to handle the data, perform checks and update the content in the database.
But it will assure you that your feature will stay available.
If you modify some HikaMarket core files ; you won't be able to update (or you can update but you will loose your modifications). That's why the view override and the controller plugin are flexible.
It is also possible to perform a class override but I can't suggest you that for the order edition because I know that the class will evolve in the next major release.
I see that the vendor order edition is becoming a requested feature.
I can't give you any schedule for the moment (because I'm still busy with the development of HikaShop 3), but I think it will be one of the next major feature for HikaMarket.
Regards,