Can you use a standard joist hanger connect lumber to an i-joist if I add stiffeners?
I would carefully fit notched blocks the height of the joist into the cavity so they make contact on all faces. I'd also put a backer block between the ribs on the back side. Then I'd use construction glue and screws to anchor everything. Mount the hanger to that.
                    ________ __
                   |     ---|--|* &lt;--- screws                    
                   |________|  |_
                   |   ||      | \
       screws --&gt; *|---||---   |  \
                   |   ||      |   |
                  *|---||---   |   |
       backer ----&gt;|   ||      |   |&lt;-- joist hanger
                  *|---||---   |   |
                   |___||___   |   |
                   |        |  |   \
                   |________|__|____|
                             ^----------filler block

I'm not sure that it meets code, but I'd have confidence in it. Heck, if your face-nailed boards held up this will.