In horizontal mode, the pre-break, post-break and no-break texts of a \discretionary
can contain any combination of characters, boxes and kerns. In math mode, however, the no-break text must be void.
Is there any objective reason for this annoying constraint? And is there any hope to see it relaxed in a future version of LaTeX? (And, let us make a wish, if \hyphenchar
could also call a macro instead of a character of the same font only...)
This is maybe possible with LuaTeX or XeTeX, but I would like to stick to standard (pdf)LaTeX (including e-TeX).