They had to block native code on WP7.x because they were on the CE kernel, knew they were going to move to the NT kernel in WP8, and native apps wouldn't have ported over.
Platform agnosticism is something that Qt exists to provide... Your point is taken that the Qt backend would need to be ported twice. For WP8+, I suppose that would involve a DX11 port of ANGLE for Qt5. Which seems to have been merged in, actually https://codereview.qt-project.org/#/c/52810/. What was WP7? DX9? Angle covers that, as well. So, I'm not sure why we're not seeing it, at least on WP8+ given that it does have native code support. https://msdn.microsoft.com/en-us/library/windows/apps/jj6816...