You have 2 ways to approach this..
Use geometry, squashed black spheres for the eyes and a bent squashed cylinder for the mouth.
Make a whole lot of small bitmaps for the eyes and mouth (512x512) and use the connection editors to make a simple rig where you can switch rapidly between the images.
On one of my past jobs we had a character that had a 2D face on his 3D body. We had separate files for the face and the body. Eventually I combined the character to one rig, but the process still remained the same.
Model out the face. Eyes, mouth, nose, freckles etc.. then make some blendshapes for the created geometry. Keep the geometry simple black shapes, since your making Lego style. Add a black toon shader to the geometry. Create a camera and place it directly facing the face. Render with the dimensions of 1024 x 1024 1.1 ratio and as a tiff seq with an alpha channel. (any image sequence format will work just so long as it has an alpha channel.) Render out an image sequence.
Now onto your body mesh. UV map the head and apply the previously created face image sequence as a texture. If your doing a Lego man type setup then your head texture will have to be a Layered Shader. The bottom shader will be the plastic color and the top color will be your image sequence.
Since your rendered it with an alpha channel you can apply this image sequence to the bump, reflect, and basically anywhere else to give you character some really nice effects.
When you feel comfortable you can combine the face rig and the body rig and be able to see in real time the changes your making to the face. Unfortunately to get the 2D face on a character and have it look 'great' your going to have to do 2 renders.
Attached is a screen shot from the DvD cover of the guy we made with a 2D face. He's the Yo Yo holding the brush and rubber ducky. The soldiers around him were done using geometry for the eyes and mouths instead of the 2D texture.
Thank for the help guys, Im just going to use some 2d objects pressed against the face and then do some blendshapes.
One final question, if I have a polygon cylinder, is there is a way to basically wrap/push a flat nurbs surface or a polygon plane against it automatically, instead of having to go in and manually push it up against the cylinder curve?