Off board Ethernet connector
Ethernet is a transmission line with a specified impedance. This style of connector will introduce an impedance discontinuity and thus a bit of reflection.

However this only matters if the line is long enough relative to the wavelength of the highest frequency that will be transmitted to actually be a transmission line.

With 100Mbps ethernet, if you use a short ethernet patch cable the whole line will behave more like a lumped LC than a real transmission line...

Therefore, it should work.

You can experiment for yourself: get a RJ45 patch cable, cut it in half, solder male and female 0.1" headers to the two pairs that are used in 100M ethernet, and check if it works.