Is there anything like a "howto" for writing such a bridge driver?
   Are there especially good examples in the tree?

any platforms machine/pci_machdep.h should help you figure out what
parts need to be implemented.
   
   It appears to me that it is a "simple" matter of implementing pci(9)
   routines such as pci_intr_establish(9), pci_conf_{read,write}(9),
   pci_mapreg_map(9), mapping memory and I/O space, filling a
   pcibus_attach_args instance and passing it to config_found_ia().

this is basically it.  it's a pity that the MD-parts of pci(9) are
not well-listed to make it clear what needs to be done for a host
bridge driver.  perhaps you could add such a list somewhere when
you are done?