well it'd at least look/act that way, that makes sense.
something like "lava below depth 5000 or so acts like aether" (and a biome fountain to let you do it cosmetically elsewhere)
iirc the engine still has a 4 liquid limit, we know they can change the colors, we know they can add effects(chill from icewater) but unless you want to waste the final liquid on basically biome specific lava thats not gonna even be in half the worlds then it can't actually be aether. without blowing their final liquid you can't scoop a bucket of lava and a bucket of aether in the same world(and you could bring them into each world so it'd just have to be special biome lava. so the planctae would just stay obsidian)
edit- doesn't need to be lava, could be special water (assuming they can make biome water glow). that might even work better