Short Answer: No, I do not believe you can force the camera's spot metering point (which is the focusing array's central focus point, proved below) to become visible while spot metering and using the auto-area AF mode.

To find out more about how I came to this conclusion, continue reading.

Long Answer: I was recently wondering how spot metering worked with auto-area AF as well. According to page 139 of the D750 manual, available here, I came to the same conclusion as you: when using auto-area AF, the camera will spot meter off of the center focus point of the entire focusing array. In Nikon's words:

Spot: Camera meters circle 4 mm (0.16 in.) in diameter (approximately 1.5% of frame). Circle is centered on current focus point, making it possible to meter off-center subjects (if non-CPU lens is used or if auto-area AF is in effect, camera will meter center focus point). Ensures that subject will be correctly exposed, even when background is much brighter or darker.

That last section in parentheses is what we're interested in here. To test my hypothesis/understanding of this passage I took the test shots below.

In this first image I tested to see how the camera metered a shot while using auto-area AF and spot metering. To check this, I made sure to place the camera's center focusing point in the shadow cast by the horizontally positioned pillow, and allowed auto-area AF to focus on the on the horizontal pillow. If the camera is indeed metering off the array's center focus point as I suspect, then the image should come out over exposed, since it is attempting to meter the deep shadow. If instead it is metering off of the center of the focus point group which the camera has selected, then the horizontal pillow should be relatively properly exposed.

As you can see from the overexposure in the above image, it appears that the camera is indeed using the entire array's central focus point to perform its metering. To confirm this though, let's see what happens if we change to single-point AF and place that focus point on what would be considered the center AF point of the group of AF points that auto-area AF selected to see how the camera chooses to meter the shot this time.

That image looks much more properly exposed for the area under the focus point, so I feel good that our interpretation of the manual is correct. To get back to the question at hand though, can we force the camera to show us the central AF point while using auto-area AF?

I have my camera set so that when I click the select button, the one in the center of the directional pad, the AF point will return to the center position. So, to test if the central AF point will highlight in auto-area AF I set the camera to that focusing mode and clicked the select button; this had no impact on the position of the locked AF points. I then set the camera to both single-point AF and group-area AF, moved the focus points away from the central position, and then clicked the select button; in both modes the focus points jumped back to the center of the array. Given these tests, it does not appear one can force the camera to highlight the central AF point in auto-area AF mode.

Based on our understanding of how spot metering works with auto-area AF, and our inability to force the camera to highlight the central focus point in auto-area AF, I am forced to conclude that we cannot get the camera to show us the position of the spot metering point while using auto-area AF. It seems you will simply have to estimate the center of the array's focusing points.

I would be really interested in finding out if there is some setting I have missed that would allow this functionality though.