Don't know how often this idea could be used, but say you have unused pre-existing 75 ohm coax already strung in the house (and running cat5 to do DtD isn't practical), and you don't want to clutter up over-the-air spectrum tying a pair of nodes together via wifi. Use the 75 ohm coax for the 2.4GHz connection. Yes, the nodes are 50 ohm impedance, but we'd want to attenuate the signals out of the nodes anyway, using passive impedance transformation circuits. That' coupled with the losses of a long length of RG59 or RG6 (better choice anyway, usually better shielding) should attenuate the transmitters to not blow the doors off the receivers. As mentioned, simple attenuators can also do the impedance transformation. A pi network of resistors would do it. A 180 ohm resistor on the 75 ohm side to ground, a 68 ohm resistor on the 50 ohm side to ground, and a 100 ohm resistor between the 75 and the 50 ohm sides. One of these circuits should attenuate around 9dB, and we'd use a pair, one at each node where the 75 ohm coax ends. Getting the funny wifi connectors can be done by taking apart those plastic antennas that came with the nodes (I'm thinking of the kind that came with WRT54G's). Remove the dipole antenna part inside, leaving a coax pigtail on the funny connector, and build the attenuator circuit on the pigtail with an F connector at the 75 ohm input/output side.
This picture shows the adaptor cable without the attenuator.
You end up with the coax being an extension of the port on the mesh node.
So instead of 3 mesh devices (1 to link you over the mesh, 1 to inject to coax and 1 to pull from the coax at the far end) you only need one mesh node.
Same concept as the injector idea but purpose built to work on standard CATV coax and get through the multitude of splitters and the like (something a mesh node probably have significant issue since most splitters are 1GHz, 2GHz if your really lucky don't think I have ever ran across a 3GHz CATV splitter)
And of course there are Ethernet over PowerLine injectors as well (though those will probably annoy the HF station a lot more)