I think this is a needed feature, the only mod I've seen so far with correct fluid physics was Create and it did so by registering all of its fluids as "minecraft:fluid/water" which had a ton of side-effects. Would some of this fit in the FluidAttributes class that modded fluids need anyway? It could have properties such as acceleration or a custom physics flag. If mods need dynamic acceleration based on an entity there could be methods to override like how water does with FluidAttributes#getColor. Patching the Entity code might be complicated though, the existing #handleFluidAcceleration method only takes a fluid tag and a fixed acceleration value.