1. Can you inject a mortar joint to fill 'hollow' area of tile?
2. Is there a clear epoxy material that would not dilute the appearance of a tile, yet 'build up' a low spot in the tile surface so water has more positive drainage for a shower?
ANSWERANSWER - I'm assuming the hollow area under the tile is a void. I have seen people try to inject a flowable epoxy through holes in the grout joints to fill voids under tiles, and I have never seen it work effectively. Normally the epoxy doesn't flow very far and the voids don't get filled.
I have never heard of or seen where you can put a clear substance over an existing tile floor in order to create a greater slope to drain than it has. I don't think it would be practical or effective or visually acceptable.