Well, on the K3, that was pretty much either it works or it doesn't, the whole family, at every size, was blank (which lead to mighty fun breakages if it happened to be sans regular, because then most of the UI was blank
The weird thing was that it only happened on some Kindles, I've never been able to reproduce this on mine, even with the exact same set of fonts file.
What does seem to 'fix' it for everyone is indeed to use a custom build of a recent freetype release. It was pretty easy on the K3 because it didn't rely on fontconfig/gtk/cairo/pango at all (except the browser), so I just had to build it, bind mount it, and enjoy working fonts after a framework restart, but you might have to jump through a lot more hoops on the Touch to avoid ABI mismatches... (If I let the K3 browser see/use my custom freetype lib, it'll make pango throw a fit, and it won't render fonts at all. I've never tried to pinpoint where exactly the issue lies, but it's most likely an ABI mismatch somewhere).