OK, it took me a while to verify that the idle state of MIDI is in fact, OFF.
hmm, I guess I don't understand what idle state means. I thought it means when there is no data being transmitted, which afaik is when Tx is constantly high, i.e. after a stop and before a start bit.
So you have R1 of about 4k7 to pull the Rx input HIGH. You want a diode instead where you illustrate R3, cathode to the Tx-B so that it definitively pulls the line LOW when Tx-B goes LOW.
Good, so I had the right idea.
I am not sure where you got R2 from, it presumably sets a sensitivity threshold for the opto-coupler.
R2 is there to improve the fall time of the photo transistor. MIDI devices usually use 6n138/6n139 opto-couplers, the 4n25/4n35 are slower and don't produce very good wave forms at MIDI baud 31250.
So I guess multiple Tx lines can be connected to an Rx input through diodes, as long as only one Tx line is active at any given time?