Transfer function of this circuit

I wouldn't trust a standard zener to protect a line running outside. Can you show the circuit? Are you sure it doesn't mean a TVS diode anyway?

You could use a single (say 6-10V) TVS on the lines as you mention.
A more expensive but more robust solution would be to use a three stage protector like this (or make your own)
It combines a gas discharge tube with a series impedance and a TVS. You get the benefits of the gas discharge (high current capability, high breakdown voltage, slow) with the TVS (fast, low clamping voltage)

From the sounds of it, the diode model you are using is the simple "ideal diode" with a fixed forward voltage. This model is an open circuit when \$V_{\textrm{Anode}} - V_{\textrm{Cathode}} < V_D\$ (reverse biased), and a fixed \$V_D\$ voltage supply otherwise (forward biased).

Start by making assumptions about the state of D1 and D2 (for example, D1 is forward biased, and D2 is reverse biased).

Your circuit would then look like this:

^{simulate this circuit – Schematic created using CircuitLab}

What is \$V_o\$ here? The last step is to check your assumptions on each diode. If the assumptions are correct, the model is applicable. If not, permute your assumptions (ex.: what if D1 is reverse biased, and D2 is forward biased? or D1 and D2 are reverse biased, etc.) Only one of the 4 possible permutations will have a consistent answer.

As a side note, the answer you get will not match what the circuit simulator gives you (though it will be close). This is because the circuit simulator uses a more advanced diode model.