How do I replace a sub-floor that goes under a wall before reaching the nearest I-joist?
Your plan, as diagrammed seems pretty reasonable. 

Does the 3/4" thickness of the 1/8 that you were proposing to glue to the center web of the I-beam bridge out the whole width of the top flange of the I-beam? If not you may want to consider using 2-by material instead. Your joist hangers will nail or screw much better into the 2-by construction lumber than they will the soft pine of the 1x8 board and the OSB of the I-beam webbing.

As you suggested you really do want to screw that filler piece in after applying the glue because you will not have access to add clamps to hold the pieces in place until the glue dries. You will definitely want to pre-drill clearance holes for the screws in the filler pieces to that the screws can pull them up snug to the I-beam webbing. Make sure to deploy screws of a type and thread size so that they bite into the OSB webbing securely without stripping out.