Jocelyn just gave me time-saving important advice:
If you're building Qt with an infix to be able to install Qt in parallel on a Symbian device, then you may run into the problem that the installation of QtMyInfix.sis fails. The error message is usually "General OS-related error", which tells us clearly what's wrong. NOT ;)
One likely source of error is a file conflict. The way to fix this is to look at the Qt_template.pkg file and comment out any files and directories that do not have your infix in the name. For example some of the qml directories are not infixed and they exist already on the device, so just comment it out in the .pkg file with a semi-colon and then it'll install.
It's a bit of trial-and-error, but for the time being it works :)