This is a very old question but to date, very relevant as this is one or the only way to do security trimming around list items, without breaking your information into multiple lists.
Although not straight forward, you can:
- Within the List settings, go into advanced and allow creation of new folders
- Even after adding this and creating a new folder, it won't show in your list so you need to add the folder into the URL (https://contoso.sharepoint.com/lists/A/Folder1/) to
- get the link for creating a "New" list item into that specific folder https://contoso.sharepoint.com/lists/A/NewForm...?source=.......Folder1/. Then
- through a page you add links to input the data in the correct place, and
- finalize by modifying the "View" for the list to "Show all items without folders" to see all items, as if you don't go into the folders which are not shown, you won't see the items within them.
Once you've done this steps you have to add the special permissions (if desired) which are not straight forward either.
- Go into the list settings, set permissions for this list, this will give you the link that should look something like this (.../_layouts/15/user.aspx?obj=%7B233130C4-4C35-4500-8352-C9F6F22081BF%7D,list&List=%7B233130C4-4C35-4500-8352-C9F6F22081BF%7D)
- Change this specific part ",list&List" to ",#,listitem&List" and replace the # with your list item/folder (If you export into excel it will be easier to find the # of the folder you want to set the specific permissions).
Hopefully this is helpful.
The most important aspect is the user training, make it clear to them that they can't add information in the root of the list, or better yet, make the root read only for everyone and that will take care of the "New" button and ensure that they only add new items through the links.