Replacing the system partition without touching the kernel? So I have an android phone that I'd like to put a clean rom onto, but it looks like there aren't any good kernels ported to it yet. I've done some reading into the android architecture, and come to the conclusion that it could be possible to simply leave the kernel as-is, and flash a new (standard) system on the device.

Is this idea at all viable, or am I missing something? And if it is viable, does anyone have any tips to how I might go about it?
It is viable, as there are custom ROMs I've seen that contains the original kernel but different system files.
There is a catch though: If for example your device ships with CyanogenMod and you want to put PA on it, you'll have to get your hands dirty with the ramdisk as each ROM needs slightly different init* files to boot.

As for installing the ROM, you'll need to put it in a ZIP file and include a META-INF folder which will contain the files responsible for the correct ROM flash, e.g. updater-script.

The full procedure is too long to mention here, so I'd say you should head over to XDA to get the info you need.