John, the problem is he has no helper. He wants to do this solo. But you have a good idea with the block of wood.
You can make a "guide" so when the mast is started up it will have something to work against. John is right that there is nothing there to keep the mast from sliding deeper into the bow. I would think that by your self, once the mast is in the slot and against the guide, the rest will be somewhat easy. just have to make sure you have the headstay right at your fingertips so you can move forward to attach.
Taking the mast down I would think would be easier. I imagine just lifting and using the guide to prevent the mast from sliding forward, you can walk it back. I would still recommend some kind of gin pole/painter's pole to use as leverage.
What am I seeing in the second image?