Hi,
My specific goal is about a dynamic OS bootstrapping or via USB storage device or via pci netcard automatically without pressing any keyboard buttons.
seabios skips any non-working entries, a blank disk for example. So you can set the bootorder to "disk first, net second" and as long as no os is installed seabios will try to pxeboot, after install it'll boot from the disk instead. Which might be dynamic enough for your use case ...
cheers, Gerd