Major Image is open ended
The spell description for major image doesn't go into a great deal of detail on how it can or can't be used. I believe this is deliberate, allowing for players to get creative and DMs to be flexible in its use. So the answer is definitely up to the DM, but lets look at what we are given to help decide if this is a potential use.
You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 20-foot cube.
A forced perspective image isn't an object or a creature so we are looking at "some other visible phenomenon". This phrasing covers basically everything, you can make an image of anything you want. The question is really if the forced perspective will be believable.
It seems completely real, including sounds, smells, and temperature appropriate to the thing depicted.
At least initially we have some support that this should work. The phrase "seems completely real" is fairly unambiguous. But the wording generally supports a single entity rather than an optical illusion so it is unclear if this will work.
A word on Forced Perspective
Forced Perspective is a form of optical illusion to make thing appear smaller/larger, closer/further away than they are. The limitation however is that Forced Perspective illusions only work from a single direction. Moving your perspective or viewpoint usually breaks the illusion.
The spell allows you alter it to appear realistic. Therefore you can shift the perspective to mirror the movements of the creatures you are trying to deceive, provided they are close enough to have a single perspective on your image. If the viewers spread out and view the image from different directions it won't appear realistic for all of them. Novak's answer goes into this in more detail if you are interested in the science.
Allowing it at the table
In the past I have allow my players to use major image to conceal themselves within a dungeon. Hiding in a dead-end hallway they cast the spell in front of them to make the hallway appear empty, thus avoiding attention from the patrolling guards.
If you are to allow the spell to work this way it becomes a major tactical advantage for those who have access to it. I personally choose to allow it as I like the kind of creative play that it encourages. However you may need to be wary of overuse. If you find your players begin to use this single spell to avoid entire encounters you may like to revisit your ruling.
Bonuses to disbelief
As written a creature will disbelieve the illusion if physically interacted with or if it:
uses its action to examine the image [to] determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC.
As I already mentioned, forced perspective illusions only work for a single direction. Therefore it is fairly reasonable to grant advantage on this check as this kind of illusion is less realistic than the typical uses of this spell.
Whether or not you choose to grant advantage is really a choice in how effective you want this tactic to be. From your question is appears that you want this to be a major plot point and something you wouldn't want the players to easily uncover. Therefore you might prefer not to grant advantage in this situation.