I've been able to mix 3D and 2D with A5 easily. I even made a mockup to test it:
Since you claim that your game is simply going to be a 2D card game, you can just use the Depth buffer that Allegro creates for you.
creating OpenGL textures from pngs
No need to, you can use al_get_opengl_texture to retrieve the OpenGL texture id, ready to use with any OpenGL code you want.
Since we're talking about just card planes, just rendering two quads(one for the back and one for the front) should do the trick right? It's not about reinventing so much in that case. Be sure to set up the proper projection before, and then just switch back to the Orthographic one.
Games Like World Of Goo also implement something similar with a goo mesh when you fill the bottle with goo.
If you're looking for that effect, I think you should just refer to al_draw_prim. Be aware that you might have some artifacts such as the one described in this thread, which is not Allegro related but just the way a quad is rendered on 2D projection.