Well, i would say, the easiest way would be to modify the HDRI Backdrop material, so that you can use your transparency in it. Since it accepts HDRI only, you would need to make your transparency a separate texture.
Then you would just need to go into it´s material folder and add your transparency/alpha texture to the material, and of course set it to a transparent material (with forward shading, to have all the inputs available).
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