I was able to get the board to stop resetting the entire system by hooking it up to 5 volts. I also finally got the SD0 overlay working and confirmed the pins are configured correctly in software. I was unable to get the card to mount, and no activity light action on the breakout board. I'm starting to suspect there is a hardware problem with this breakout board. The overlay is set up to use all of the data pins of the SD card, not just the two that are broken out on this board. I'm going to order a simpler SD breakout board for the next round of testing. I was very frustrated with the lack of progress on this, but I have the software sorted and am happy with that. I'm hoping to have an SD card working over GPIO shortly after the new SD breakout board arrives. If that's the case, this project can move forward onto the software development stage. As much as I want to skip ahead to hat and start making games and programs for this, I don't want to get distracted and put off the hardware aspect, as it's the most important part. 

The problem I was having with the software was my own dumb fault. I put the sd0 overlay file in "/boot/" instead of "/boot/overlays". That caused it to fail to load. I was also playing around with different overclock settings and such, which I reverted to the original settings. The pins are now configured properly and the OS tries to mount the SD card over SDIO at boot, but fails and starts the OS afterwards. 

I still have the 512MB SD flash chip I'm not using. It uses the SPI interface, which I have no idea how to use with the Raspberry Pi OS as a mounted drive. I may just use the SPI interface for saving configuration files and data I don't want the end use to mess with. I'd include documentation and ways to modify it of course. 

Still many important decisions to make, but progress has been made. The first step after getting the SD card working over GPIO will be performance testing. I need to know just what I can do with this card. I'll start with simlpe benchmarks, then move onto real word testing, such as streaming high bitrate video files from it. Read speed is most important to me for this project. I don't have a working SDIO SD flash chip yet, just the SD card itself. The datasheet for the chip claims class 10 performance, I believe. I will just have to get a similar SD card for initial testing an confirm with the flash chip when I get one working.