The splitter would need a basic understanding of whatever protocol you're using so that it could change the direction of the transceivers appropriately, but other than that a splitter is just a handful of transceivers and that's basically it. Maybe some isolation between downstream ports if you wanted to get fancy. You could do it with an 8-pin micro, since the upstream transceiver's RO line can be tied directly to the downstream DI lines, and the downstream RO lines can be ORed into the upstream DI line. Then the micro just needs to manage the upstream and downstream DE/RE. That's only four IO lines, so you still have one or two left over for LEDs. The micro just has to know when to expect a slave-to-master message and flip DE/RE, and then when the message has ended so it can flip them back. Come to think of it, you might want to monitor the upstream and downstream channels between the upstream and downstream transceivers separately, so that either means an MCU with two UARTs, or maybe a soft UART if the bus speed is low enough.
Of course that's only suitable for the application you described, where the upstream port talks to all of the downstream ports and the downstream ports only talk to the upstream ports. If you needed to be able to communicate from one downstream port to another things get a little more complicated.