You know, I did not read your post. Okay, so you want to make curved lines to spell 2017. For my method to work, either your background or your line/letters should be a solid color (I think there is a way to make it work even if both are not solid, but I'd have to do some testing).
I don't know the best way, but assuming your letters are a solid color, here is what I would do:
1. Create the letters as the will be at the end. You will NOT be changing them, so you don't need any waypoints on them at all.
2. Place the letters in a group.
3. Within that group, create one or more shapes (read to the bottom and adjust your shapes appropriately)
4. Make sure the shapes are ABOVE the letters in the group, and that they all have the same color as the letters.
5. Change the "blend method" of all your shapes to "alpha over." Now, wherever you shapes are, they will make your letters invisible.
6. Animate your shapes so that at the start, they cover the entire line image, but reveal the letters as the pen writes them.
See this tutorial if you need more help:
http://wiki.synfig.org/Subtracting_ShapesIf your line is colored more complicatedly, but your background is a solid color, you can do the same method, only make your shapes the color of the background and set them to "onto."
UPDATE: I was messing around and found a way the works all the time. Make the masking shapes like before, but this time change the blending method to "onto" and make the color anything with a 0% alpha. That way, the masking shapes will make your letters completely invisible, regardless of the color of the line or the color of the background.