Can MTP be configured to show the root? I have a rooted device running Android 6.

When I connect it via USB, I can browse files via MTP, but it is limited the the /sdcard folder, for instance I can see Downloads and DCIM but I can't access /system/app.

QUESTION: Is there a setting or trick that would allow me to access / via MTP?

Very technical procedures such as recompiling system programs are OK too.
The simple answer is no, you can't... You are not really looking at /sdcard but at a database of what that card contains provided by External Storage and possibly Media Storage. MTP has replaced USB Mass Storage (which would allow this) for a variety of reasons, many are security reasons, and they are encoded deep within the Android source code in multiple areas. MTP is specifically designed to only give access to user files (/sdcard and /storage/{volname}) and nothing else. I don't think you will find a way, root or otherwise, to this without heavily modifying the base code of Android