You want me to display moving belts for some players and not for others? Unfortunately, I don't believe that is possible using the tools currently available to modders.
Very few of the modding tools allow for things that only apply to individual players. My mod achieves what it does by actually changing the belt prototype (the assets that get loaded when loading up factorio) so it has an animation speed of 0, and isn't something that could even be changed in-game, let alone for specific players.
There really is no easy way to do what you're asking if it is possible, and certainly not achievable by a very simple mod like this one (This mod is only 12 lines of code, 6 for the beacons and 6 for the belts). All of my ideas are a bit of a stretch and I don't know if any of them would be actually workable:
- Draw over all the belts and beacons with a GUI overlay (like what FNEI uses to draw on the screen which can be shown to specific players)... though this normally moves with your screen. I haven't played much with this modding feature and don't know if it could be made to work. I'm not sure how players would know the direction of belts if I'm drawing over them all with black bars.
- Hack one of the radiuses (like transparent color displayed by beacons or electric poles to show you their range) and maybe make it less transparent and have each belt you place down also place down a tiny invisible beacon... and then also figure out how to make the game show those ranges 100% of the time?
- Maybe find one of the debug options that could be manipulated into intentionally covering up belts?
These would all be pretty hard (if even possible) and they would all look pretty bad and would involve somehow trying to draw on top of belts.
I think your best bet is to put in a request with the developers in the factorio forums: https://forums.factorio.com/viewforum.php?f=6
