I have rolled several of my own DirectX-only engines, so I'm guessing that no, most people probably do not.

I always wrote them so that all DirectX and Windows specific code was abstracted behind platform-agnostic generic interfaces. So, in theory, OpenGL support would be as simple as coding an OpenGL implementation of my generic graphics API. Probably a day or two's work, but I haven't had a compelling reason to do the OpenGL implementation yet.