Under the settings for the light casting your caustics, you can assign a Light Pass ID. You can then select this ID as a pass in the Render Target node settings and render direct and indirect lighting for that specific light. These will appear as separate layers in your EXR file and the indirect pass will contain the caustics. This isn't a pure caustics-only render (I don't believe Octane offers that), but if you pick a light that is only targeted for caustics and only accept that light on the objects you want to have caustics, it should work. It isn't perfect, but it is OK.
However, since you are in Houdini, I'd recommend looking into rendering the caustics with the built-in Mantra renderer. Mantra is capable of rendering FAR more detailed and less noisy caustics, much more quickly. That way, you can avoid using the slow, noisy PCM Kernel and just comp in the Mantra caustics pass with an Octane PT Kernel render. Mantra is slower, but you can turn down the samples for the beauty pass and get a fairly quick caustics render which you can easily isolate by creating a pass specifically for the caustics light diffuse/reflect/refract channels. The diffuse caustics channel is pretty noise free, even with samples set very low (the reflect channel is another story). The caustics light gives you unrealistically bright caustics out of the box, but it gives you so much control in post, you can tweak to taste.
I'm including a hip file so you can see both setups. Also have some screenshots below in case you can't open the hip.
Hope that helps!
Joe