In the custom cubic menu, there could an option for generating the farlands. The menu would be on its own separate page and will simply have checkboxes for generating the farlands at all, or in the x, y, or z axes.

Is there not a modified jar that does generate farlands?

There is but it requires using that modified jar.

Would there be a way to integrate that modded jar into the regular jar and simply have a checkbox to select or deselect farlands? That's pretty much the only way I see to make it any easier.

Farlands preset could be neat. You mean all messed up before they fixed the bug, right?

Configuring it for separate axis isn't going to happen. But there will be option to choose a noise generator that has that issue unfixed.

Fixed in OpenCubicChunks/CubicWorldGen@c511846