Actually you need two things.
1- A window with masking support
2- Runtime gizmo system. Because (probably) you need to rotate that model sometimes. (I am thinking this kind of stuff on main view. https://help.autodesk.com/cloudhelp/2023/ENU/3DSMax-Basics/images/GUID-EF5722FC-D0E1-42D2-A12A-34C312F9F73B.png)
My solution doesn’t support masking at all.
But I would use ScreenCapture2D if I was in your shoes.
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Put your model to unreachable and unvisible location which has a black background (there are some background sample for virtual production usages)
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Capture that view and use that captured texture in a material.
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Mask background
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Put it into to a widget
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(BONUS) if you need gizmo, write your mechanics to rotate that ScreenCapture2D.
For performance, you can toggle ScreenCapture2D’s capture with blueprints. When you don’t want to rotate it, just disable capture and only open it when you rotate it.
Other than that maybe you can try to integrate ImGui to Unreal for creating new windows , gizmo system and model rendering on Android. I saw some ImGui usages on Android but they weren’t related with Unreal.