Does the OpenType font format provide support for extensible arrows (as in
\xrightarrow)? If so, is it true that it would be
unicode-math's job to make this functionality available in TeX?
This functionality, besides being useful in itself, could possibly have the following interesting by-product. If fonts made the tips and tails of various arrows available as separate glyphs, then drawing programs (including TikZ, despite its claim to not be one such) would be able to use them to create graphics with arrows matching those in the document's main font. (My particular interest is to create commutative diagrams, which are really just like any mathematical formula, and should look like so, but often require a general purpose drawing package to be drawn.)
For things to work fully automatically, drawing programs would also need to have access to some parameters, such as the line width of arrow stems. Does OpenType accommodate this?