Platforms should provide the necessary low-level APIs to hook into their accessibility- etc... features without having to use their high-level UI frameworks, and they should provide lower-level services like high-quality text rendering.

Mostly this already works, I can open a standard file dialog, or copy data into or out of the system-wide clipboard. If there are any APIs for accessibility missing (e.g. for hooking into screen-readers), I would consider that a bug on that particular platform. There's no reason that applications which use the system UIs should behave any different from applications using custom UIs.