Interfering or not...

[socal lurker]

This scenario was covered in pre-match recently. Referee advised what to look out for. If the keeper has moved in accordance with the ball flight and made an attempt to save it then there is probably no impact from the player standing in an offside position. So do nothing. (Which I think is the scenario outlined in the original post.)
However, if it seems that the keeper has not been able to react, then it could mean they were un-sighted. I have 50 percent of the info and and should flag the offside. The referee has the other 50 percent based on his positioning and he can ultimately decide if there has been an obstruction of the view. If not he can overrule my flag.

I think the right protocols for his vary in different places. In the US, if we don't have enough information to determine OS, we are expected to stand still. That is the signal to the R that we need to talk. And at that point the pieces can be put together to determine if there was OS. (And Rs know that they should not signal the goal until they make contact with the AR, who will be moving back up the field if he believes it is a good goal, flag up if the scorer was OS, and at attention if there is a question about the validity of the goal.)