Awesome indeed Morkl!
The holy grail would be to find something that works without rooting, but rooting is so easy and it looks like these font issues can be fixed (relatively?) easily by modifying the Sony apps.
Do you know anything about dissecting shared libraries? /system/lib/libreadercore.so contains a reference to res:///userStyle.css, but I haven't yet been able to figure out how that is used.