2x12 and 3/4" Play for stringer ledger?
You could attach 2x material between the cords of the I-joist that the stairs will attach to. 

Attach a piece on both sides and bolt them together through the web of the I-joist.

Then attach your joist hangers for the stairs to the 2x material in the middle of the joist.

The other option is to add an LVL sistered to the I-joist on the stairway side. Giving you plenty of material to attach to.

Good luck!