sounds to me like you are doing everything just right. As others have suggested, indirect play can help. But basically siamese and orientals can be like that. I had one once who did not emerge for several weeks, and even after that, she would hide at the slightest hint of vigourous activties or visitors. She did come around, and ended up being a real snuggle-bun, so you have that to look forward to. Siamese are human-oriented cats so it will all come right in the end. In the mean time, just keep doing what you are doing, don't look directly at them when you are 'enticing' them, and be patient. Sounds like you are doing a great job.
If there are vast expanses of open territory between pieces of furniture, you might also consider providing the odd 'cave' or 'covert' within the gaps. That way they can start to move around the place more confidently. Don't let them outside until you have their confidence though as you may never see them again!