Well, keep in mind that IC has a pump machine that will pump lava and put it into cells -- I know it was bugged in 1.0 though, does it work in 1.15?
True. The Pump needs to be able to pump pure liquids if it can't already. If a bucket or cell is placed into the output slot then it should fill it, as per normal IC. If the slot is left empty then it should output the liquid (without visibly filling the slot). In neither case should the pump spill stuff onto the ground, just stop. In this way, if the player puts in an empty bucket, it'll fill it and stop pumping until another empty bucket is added. If the player has Buildcraft installed and connects regular pipes, it'll take empty buckets as input and output filled buckets into the pipe. If a waterproof pipe is connected and the slot is left empty, it'll output pure liquid into the pipe. No artwork is necessary from IC since no liquid output is possible unless Buildcraft is installed.
Actually, since Buildcraft has its own Pump, the most important thing is simply that the Generators be capable of accepting liquids.