Hi, Folks -
I'm the developer of the TI-99/4A 64K ROM cartridge board. Rather than bore you with all the details, I'll put put my Youtube channel and website out there and let you look for yourself.
www.hexbus.com - This is the website that has pictures of my 64K ROM cartridge board, prototypes, multicarts, and many other things that I've done with the TI (Hardware Projects section). Currently accepting pre-orders (link on website) for the boards. They can run 8K to 64K programs, and even can be used as a multicart (seven 8K games, or up to 60(!!) if you hack them.)
http://www.youtube.com/acadiel - Links to videos about the cartridge board, some of the retro games authors are making that use it, etc.
I'm currently running a preorder for a batch of these, and I'm trying to reach the 250 mark - where the board are approximately $10/ea. I'm at 115, and need to get to 250 by 03/01, so if you know anyone who wants one or more of these, please send them my way.
This is only a hobby, I'm not making any profit off of these - I only want to make homebrew tools available to the TI community.
Any questions, please let me know.