I used to live off of Sorrento, up until 1992. Even then the Fanno Creek crossing was a topic of discussion.
Here's a link to google earth
that shows the trail which goes over Greenway Blvd, which is about a half a mile from the Fanno Creek Trail crossing being discussed. I had thought that this type of treatment for the Fanno trail would be nice, and be possible in terms of getting money to pay for it. I suspect that doing a grade separated crossing for the Hall Blvd roadway would be prohibitively expensive.
Another example of this type of crossing is on Cedar Hills Blvd., here
A problem with both of these examples is that they show the road surface below the grade of the surrounding terrain. For the Fanno Creek trail crossing, a bridge would have to arch up over the trail. And it would have to allow sufficient height so that it would not get hit by a passing truck. That happened once to the Greenway Blvd. crossing, which is possibly why they have the orange bridge height sign on it now.
And a bigger problem with this is that it would syphon money away from a possible fix of Crash Corner, aka Oleson/Scholls/Beaverton-Hillsdale Highway.