Just stumbled upon this thread. Can't wait to see this completed. I loved the 5200 release which I picked up as soon as it went on sale. I'd do the same for this version as well!
Any further progress since December?
I also stumbled onto it, as I also own the 5200 Tempest and have seen the protos for this well-done cart, yes, while it took more than the onboard 4K (like Pac-Man 4K did) to get it done, I have already acquired a DEAD-STOCK (brand new in box, only taken out for photos) 2600 Jr. with the short rainbow on eBay and plan to get it a/v-modded as soon as I get it, but will the ROM work with Harmony, as I plan to get either the SD or micro SD version of it and fill it up with some of my faves from the good-old days. But it was THIS version of Tempest that convinced me to get the NOS 2600 Jr. I also saved this thread in my bookmarks under "Atari Forums Sites".
Now, can someone do a 2600 version of Donkey Kong (and Donkey Kong Jr.) that is far superior, and is no more than 4K, to the one that Coleco butchered (a tactic Coleco used which is well know to us Atarians to lure us to get a ColecoVision).