I have seen the Adobe Documentation and this functionality does still exist. I believe the issue is that there is a new "privileged context" (or something similar) and this needs to be added/created in the code. I am not sure how exactly to do that.

For this problem, I have decided to just drop the FDF interface entirely and use the Adobe API to add values to Adobe Form fields directly. That will involve re-arranging code more than I would have liked, but this approach works fine in 64-bit Adobe.

If anyone does know how to set up this "privileged context", please respond to this. That is the true answer to this question.