@MorningSong: Yes, I'm aware of that (it's hinted at in the doc, actually).
It's not particularly pretty/easy to tweak automatically without adding a bit more overhead, and, depending on the metadata of the font, it might just not be possible at all. Hence my conservative stance to do nothing particular, and rely on correct metadata

.
You might be able to tweak things a bit, without having to tweak the fonts themselves (but in case you do, I usually use FontForge, and it did the trick quite well):
Look at linkfonts/etc/local.conf (& the template file). Look at what's done with Caecilia, where each style has a different family name: they glue all of these together under one single family name, while fixing the styling for each. You can try to do the same with your 'broken' fonts, and see if it helps

. It's fairly straightforward, but it probably doesn't hurt if you have a bit of experience regarding dealing with fontconfig on a Linux system

.
@Prestidigitweeze: There's no real issues left with KF8, it just takes a good 5 minutes to boot the first time, and, like MorningSong is experiencing, might be a bit more picky about the fonts metadata. (I'm speaking of KF8+Fonts hack, I'm leaving out the usual quirkiness of the KF8 renderer on the K3

).