That's a cool idea. I wonder if the weight of the needle pushes the fluid around as it spins. That might make some trippy designs, especially if you have a vertical player.
Unless you're a DJ. In which case you want to have a little extra pressure on to keep the needle from skipping while manipulating the record.
Most DJs are digital these days, anyway, so you don't have to worry about wear because there's no actual music on a timecode record. They contain nothing more than digital signal so that the DJ software knows where you are in the song. When it wears out, simply replace it with another one.
I’m not sure about fluid filled but I have some that filled with other stuff like glow sand and pixy stix and it’s mostly stationary during play. I would imagine liquid is more mobile to be fair. I always wanted a liquid one but they’re always one of the toughest to get