no, I still haven't made a midi thru circuit.
I've been trying to "fix" my midi problems by creating those preset/randomisation/pattern generators, so I only have to control some "live-tweaks" and keep midi communication to a minimum (next to this, the 5-axoloti case has been on a hold for a while as I was writing modules to use in a band). Though, I admit, this morning I've been thinking to take on the project again.
As for the higher baudrate.. I wouldn't know, haven't tried it myself.. theoretically, there shouldn't be a maximum baudrate as long as the components (both axoloti and optocoupler) can handle the rate and still get from 0 to 1 quick enough. In the end, the axoloti will set the maximum, as it probably won't be able to send any faster after some point. The optocouplers might get really fast I guess, if you're willing to pay for it.
Using the digital IO's for communication would also be an idea indeed. This doesn't need an optocoupler as you'ld use voltage to send over the data instead of the current. Though, when connecting multiple receiving boards, you might need a voltage buffer again to keep the voltage high enough when splitting the signal.
As for the midi over usb. I assume you should be able to send way more midi data without choking it. As long as you can make all the connections that are needed, this might work. My problem is that my midi data is coming over from usb as well, so I cannot also send it from the first axoloti over to the next using usb (at least not from the one that the midi controller is connected to).
It would save quite a bit if we could at least use an USB-hub to split the signal to multiple axoloti's, but for that we need to wait on johannes to make that possible. I think this would fix problems for a LOT of people.
Though, this is giving me an idea for some dedicated IO modules to send over data serially. Just like midi, but then with customisable data resolution and the possibility of using different IO's for different midi data types or channels. This will use up the digital IO's, but as long as you control it with midi, this shouldn't matter. Also, this way you can divide up the data to send it over more quickly, eg. using a seperate connections for midi notes, clock and controls to keep their latency as low as possible.