MathML Overlays

New topic for my forum: Number Crunching, for all the mathematics-related ideas I have.
MathML Overlays
Had an interesting idea, and decided to go after it: what would it take to overlay one MathML expression in parallel markup (see Chapter 5 of the MathML 2.0 spec for details) onto another?
The proof-of-concept is a two-file zip included by link here.