How can I make my own custom OSX keyboard with autocorrect relying off a personalized dictionary?
Custom Keyboards are commonly made with the Ukelele app.  For input to work right you really need a Unicode font for the language.  Spellcheck, etc is not connected to either the keyboard or the font, but requires .dic and .aff dictionary files.  Info on how these can be made is available at

http://www.openoffice.org/lingucomponent/

Whether a custom spellcheck dictionary will also do autocorrect I don't know.

For Kouri-Vini you don't need a special keyboard or font, just the dictionary.