Hi,
I am trying to retarget between A and T poses and bumps into the same problem. Don’t have much to add, except i am also interested in a solution 
The problem is that retargeting a base pose is necessary and yet isn’t currently an easy process.
The documentation describes how to retarget a base pose manually, but you have to do this with your mouse, which is very not precise, unless you spend hours tuning every tens of degrees on each bone. And if you’re not a experienced animator, even then you end up with a poor result.
So it would be super nice to have a way to T Pose the ue4 mannequin, either with a button, or at least by adding a clean T pose in the animation starter pack for example, and a way to quickly make that the reference pose for the ue4 mannequin.
Cheers !
Cedric