From a testbed perspective, it would be good to drive a LOT of traffic
through the routers of all sorts, while watching bismark go.

So one thought is to drive them from an existing box(es), close by, from
multiple network cards from a given machine or set of machines.

Is there a machine available that has a few spare PCI or ePCI slots?

are there some extra network cards lying around? gigE pci and pci-e cards
are very cheap nowadays and there are also wireless pci adaptors of various
sorts....