Going to the last option in the most popular page, I can customize the rules and choose a position from the list, but what I can't do is make up a position to play that isn't on the list.
Board Editor for the custom variants

Yes you can mess around on variants-test, but it's far from ready as you will notice.
Nice, for the moment you can use a tool i worked on with my team. We named it Board Painter. You can it here: www.musketeerchess.net
I was wondering if we could have a board editor for variants, so that we can create custom positions for the custom variants instead of being limited to the existing positions?