Currently the only way to define geometry for a Model item is to define a mesh source file.  This means that you need to know ahead of time what the geomery will be or create a mesh file at runtime.

There should be an API to define your own geomery at runtime that uses the correct channels.  This is similar to QSGGeomery in Qt Quick or QSGGeomeryRenderer in Qt3D.

This also enables the usecase that a mesh changes dynamically each from without the use of a shader.  This can be expensive so it's a specialized case, but should be supported non-the-less.