There appear to be a wide variety of commercial products that do precisely "contact closure over ethernet".
Of course, being industrial products, they bear an industrial-strength price.
Assuming that when you say "ethernet" you mean routing data packets through one or more switches, hubs, etc.
Otherwise, if it is a single-hop from point-A to point-B, some people simply use the un-allocated pairs of the Cat-x cable.
There are also gadgets that "break-out" the "spare" pairs from a Cat-x cable in ethernet service (which uses only 2 pair of the 4 pairs)