Well, today was interesting. I went to four separate "hoses and fittings" type places, and the situation stumped them all. Nobody knew where to get another of those little inserts - or indeed what they are actually called. At one place I lucked into some old-timers and they said something along the lines of "yeah we've seen these before but don't know where people get them". So yeah...
At the last place I more or less gave up and asked them to make me a new upper line - but then the techs were all excited about bringing me some kind of three-adapters daisy chained together contraption. In the ludicrous heat that we have here I didn't insist and just went home - where I realized that they gave me a 3/16th to thread into the 10mm female fitting which threaded together but badly - and the whole thing added too much length to the line anyhow. All of these places advertise that they'll make brake lines while you wait, including for custom applications, and I was a bit surprised they didn't offer me any kind of "this is the right way to do this" solution. Anyway, I'm going back tomorrow to return the adapters and see if they could just do something with the line.
The part I don't understand is why it's not possible to just switch out the fitting. It seems trivial to me, but perhaps the whole problem is that everything is metric and local businesses just don't stock that stuff. Can I just order it somewhere?
The lines are in excellent shape, seems weird I can't just swap in a new fitting.
I threw the lower line on there, still running my original upper rubber line. Things are still pretty spongy. Mostly I'm practicing my bleeding approaches, trying to get some sort of an improvement. Can't tell if it's the remaining rubber line, me not being able to get the air out, MC starting to fail, etc. This bike has never had a decent-feeling front brake - even after I bled it with a commercial vaccum bleeder at Moto Guild, and I'm trying to change that.