My custom mapper type doesn't work in multiverse. What's the correct way to create a mapper type?

Pastebin :
This is the MVector3 mapper type that I want to add to my mod. Copied from BraceCube mod.
I found that BraceCube mod has a good mapper type and added it to my mod. Everything worked well in single-player mode. But when I enter multiverse to use level editor, the value set in the mapper type cannot be saved to bsg file, and goes back to default when I load the saved machine. The same thing has happened to both my mod and BraceCube mod.
The document for custom mapper type is not quite clear, and doesn't contain an example to show how to create an own mapper type correctly.