Well first. Don’t model in the T pose. That went out years ago. It is better for shoulder and arm pit deformation to be in the A pose, which is where the character is most of the time. This is why the meta rig is the way it is.
If you need a T pose on the model for Unity, do that after you have completed modeling and rigging. Then simply make that the rest pose by posing the model into that position and using Pose/Apply Pose as Rest pose.
Then when the model is animated and its arms come back down into a natural position (the initial A pose) you will have far better deformations around the shoulders.
Of course it should be noted that mocap data comes in as a T pose. So there are uses for the T pose. But you should not use it for modeling and rigging. Create a new rest pose for that.