To make a long story short:
The bad news is I still haven’t come up with working design. The good news is that I think my problem is with my MIDI interface (A Radium49 keyboard) I tried several opto-couplers, and I couldn’t get the 6n13x’s to give the slightest hint of a signal on the other side. Admittedly, I didn’t find a 6n138 at the lab, so all I tried was 6n136, 6n137 and 6n139, which should all be good for MIDI. This was something I double checked with an oscilloscope, but no reaction.
I also tried a CNY17-3, which did work, but only barely (Sometimes copied the data corrupted)
I think my Radium has too much resistance, so that the current it can gives cannot drive the LED in the opto-coupler. (The alternative is that I just suck and should give up :p ) Some day maybe I’ll check inside. But my next attempt will be to find another MIDI device to check if I was correct.
Until then, stay tuned.