Hello,
Disabling bConstrainAspectRatio is intended to work with the Scene View Extension, to allow you to lock both aspect ratios.
The function you are looking at in the Scene View Extension is correct, for modifying the projection matrix.
Scene View Extensions have another function “IsActiveThisFrame(…)” that controls if the other functionality of the SceneViewExtension.
If you need more information in “SetupViewProjectionMatrix(…)”, you can look into engine modifications to add a parameter of the type “FMinimalViewInfo”. As a starting point, please see where this function is called in “ULocalPlayer::GetProjectionData(…)”
For engine modifications like this, we can offer information about existing functionality, and we leave the details of the implementation to you.
Please let us know if this helps.