It is possible to develop drivers for new PCI cards as they become available. Drivers being dynamically loaded, no kernel relinking is necessary. You just need to drop the driver binary in the proper system folder, and you can then start playing with it.
The case of disk drivers is a bit more subtle. If you are designing a driver for a device you want to boot from, you obviously cannot rely on the dynamic loader in the kernel (our kernel itself is loaded from the boot device). This is why we have the drivers for all the boot devices statically linked with the boot ROM of the BeBox and the kernel.