2D antialiased graphics using OpenGL

I'm not sure if this is the right place to ask the following question, I'm
sorry if it isn't...

When doing 2D graphics using OpenGL one needs to draw front to
back using GL_SRC_ALPHA_SATURATE. I don't think this would
always be possible, so one would want to render offscreen and then
combine with what is already in the framebuffer. One could use a pbuffer
for this (or a pixmap when using GLX), but it seems to me that using an
auxiliary buffer would make sense for this case, i.e. the offscreen drawing
is related to a visible framebuffer area. Would using auxiliary buffers in
this
way benefit Cairo with the OpenGL backend? Auxiliary buffers don't seem
to be supported much, yet they seem quite useful to me.

--ms