Skuriken Particle system: Start Speed Curves on per axis available?

Hello World,

Here's a breakdown of what I'm trying to do: In Shuriken, I used a box shape emitter to create a cloud bank with billboards, standard procedure. And used "Infinity" on the "Particle System->Start Lifetime" field, and trying to make the clouds move slowly and randomly by putting a random constant between 2 numbers in the "Start Speed" field.

Here's my problem: Using Shuriken, I'm trying to only move the billboards with infinite lifetime in only 2 axis (x and z). Can "Start Speed" be broken down to separate axis curves or constants?

I can't used "Velocity over Lifetime" because of the "Infinite Lifetime", and it also crashes Unity 3.5.

Cheers, Felix