Pixel by pixel this is going to be extremely slow. However, all you need to do is create a "cache" bitmap, render your tiles to it in regular 2D, then use a perspective transform (al_perspective_transform in 5.1) and then draw the cache bitmap. It's basically free mode 7 using the 3d hardware.
Here's an example where I do it in my game:
EDIT: Forgot to mention, I do a rotation along the X axis as well as the perspective transform.