font = Font.open("MyFont.ufo")
glyphA = font.layers.defaultLayer["A"]
point = glyphA.contours[0].points[0]
point.x = 404;
assert point.x == glyphA.contours[0].points[0].x
Unfortunately, it's not clear from the article what the use case is. Most of the time I've worked with fonts, they're treated as immutable values.
If the idea is that a font needs to be built from a serialization format, then an alternative approach would be to eliminate the python mutable interface and replace it with a build function that calls the underlying Rust to build a Font. That way Python never needs to deal with mutating a font.
Rust lets you take unlimited numbers of references to an immutable value. That's not a problem, other than defining lifetimes if you pass those references around and/or hold them in structs.
The problem arises the instant you want to mutate the value behind a reference. Hold just one active immutable reference at the same time, and that won't be possible. Note that mutable and immutable references can in some situations exist in the same block thanks to non-lexical lifetimes.