Okay, I have man on the inside Well, I know someone at the dealer. I've asked the question and apparently it's corrosion on the inside. There's some kind of central strengthening bar inside the heater core / matrix that I guess helps keep the thing rigid and give it strength; this is the cause of the corrosion which slowly builds, and then causes the flow of water to the drivers (or passengers) side of the heater core to be blocked, for reasons you've already established earlier in the thread (it's vertical and I guess clogging happens from the bottom upwards).
Mitsubishi have fixed the part so this strengthening bar thingumy doesn't cause the issue any more; but for cars affected, I guess there must have been a batch of them with this manufacturing fault. Not all Mirages, but some.
Think that's as good as I'm going to get without hanging around the garage in a couple of weeks to see what they fish out from the dashboard! But hopefully that gives you more to narrow it down.
I suppose the corrosion cleaner you're experimenting with earlier in the thread might help shift the deposit and unclog it; but I guess if that was a proper fix, dealers would do that, rather than spend a day on labour. From what I know, warranty work pays - but not that well - so if there was an easier / quicker way I guess they'd do that!
Said earlier, but I'm told it's a day of work to do the job, and can run into two days depending on quirks and whether the mechanic has a good day (or a bad day) with getting everything out of the dashboard, as the steering column has to be dropped and I guess it's just a humongous beast of a bit of the car to lift out of position, given all the things that will be hooked into it, and need to be put back. Plus imagine if you forgot to click something back into place ... could undo a few hours work if you only discover it later on!
Hope that helps guys