It is done.
I went with an older Pioneer headunit I had from a previous vehicle, Pioneed TS-d1002r 4" speakers mounted under the dash up front and Pioneer TS-d6902 6x9s under the rear seats.
I mounted the headunit under the dash with a universal mount, and attached the remote the the top of the dash with Velcro.
I made small enclosures out of 4" PVC sewer caps for the front speakers and epoxied them to a 3/4" plywood face that I bolted to the underside of the dash. I tried running them without the enclosures and they distorted at higher volumes / lower frequencies. I sealed the enclosures up tight and there is no distortion even though I am not running any high pass filters. The enclosures are about 0.5L so they are prettty small and control cone motion. They do not interfere with my legs when driving and are just a slight annoyance when getting in and out of the car.
The rears are mounted on a single piece of 3/4" plywood that I fitted under the rear seat and covered in vinyl.
So, how does it sound? OK. It is about 500% better than when I had no front speakers, but it would be better if the fronts were up a bit higher and had more midbass. Generally it sounds better than most modern cars factory audio, but it is nothing to write home about. The Pioneer 6x9s put out a VERY impressive amount of bass, but no one would confuse them for an amplified sub.
Overall I am happy with the results. I have decent sounding tunes that do not take up much precious interior space. Pics for those that are interested:
Fronts - You can see the pass side just under the dash and inboard of the rollbar. The drivers side is hard to see: