Alternative to optocoupler? Other switching devices?
It doesn't sound like you need very precise control over the exact resistances.  Are you just trying to see at what point you cause enough of a load that the line drops?  In that case you can just hook up each of the three resistors with a mosfet.

Here's an example:

Essentially, you have your microcontroller pull CTL1 high to connect R3 (oh jeeze I mixed these up), CTL2 high to connect R2, and CTL3 high to connect R1.  Assuming that ETH_IN+ and ETH_IN- are at voltages far enough apart that the control voltage relative to ETH_IN- is "large" (i.e. more than a volt or so) this will definitely pull the rail low through the resistor.

That being said, it will also slightly affect the resistance, but not substantially.

Hope that helps!



Note that if you're willing to do tiny surface mount parts you can get super cheap mosfets (about 14 cents each on Digikey for one that can have a drain of up to 350mA continuous).

Or about 75 cents for a through hole one.