You have a true tabernacle, and inside the cuddy is a compression post. Mast nut would not work in this configuration. Keep the second pin as a spare. It's not needed when the mast is raised and, if forgotten, will get in the way when you try to lower the mast.
A hightfield lever, as Baysailer suggests, would require additional parts, and also some differences in the way the forestay is set up. (I'll let him explain that kind of setup further).
I found this link
http://www.glen-l.com/free-book/rig6-2.html showing a setup with the shrouds set up like your boat, but a turnbuckle for the forestay. For such a setup, you set the pins for the two shrouds to their eventual position - raising the mast and pulling it forward will then tension the shrouds (the forestay needs to be tensioned separately).
One way you could tension the forestay would be to run the jib halyard to the shackle shown in your picture. (If it's not long enough, you'd need to extend it). You then belay the other end normally.
You could then try to have someone pull the middle of the halyard back, to tension it. The forestay should become slack and you should be able to move it to another hole.
If that doesn't generate enough tension, you could put a block and tackle between the jib halyard and the shackle. That's the system used on some smaller dinghies (there they don't even use pulleys, just a lanyard that's run several times between two eyes.)