Manual tuplet breaks and e-reader

[Пронин Илья Сергеевич]

I'm writing document generator for using with e-book readers. I've a
large database with score data and template for a document. So it's
intended for creating PDF without manual editing score data.

Generated pdf fits 6in x 4.5in page nicely, but lilypond can't
auto wrap tuplets. Documentation says I need to write something like

\times 2/3 { ... \bar"" \break ... }.

I want to replace it with

\times 2/3 { ... #(some-func) ... },

where some-func calculates absolute offset and inserts break where
needed (X-offset is near 4.5\in or something like that).

What I need to know to write that function?

------
Eli