I was wondering about something. I'm trying to think of ways to test banners without bricking, and I thought I'd ask if this was possible.

We take a copy of an existing IOS, modify the /dev/fs driver to redirect to /dev/sdio/slot0? So instead of reading the NAND, it would read your SD card?

Then you could mirror your NAND filesystem onto an SD card, use GeckoOS or something to "reboot" into the SystemMenu, but changing the IOS to use your new one. This would mean that you could technically run you're Wii off the SD card.

Would it work?

Only if you completely replace the FFS module in the kernel.
You would have to reverse the existing one first to understand all messages it handles and somehow rewrite that then to use the sd slot. You can't just use the SDI module because it isn't even loaded when the kernel loads. The kernel uses the FFS module to load almost all modules from nand. you would therefore need to implement your own sd host controller driver and write the FFS module around that.
It's probably easier to just wait for BootMii :-)

I wrote most of a System Menu (not IOS) hack to load banners from a USB Gecko instead of NAND. It works, and I could test a banner with no fear of bricking, but it's slow and I cba to make it faster and/or release it or whatever. Contact me if you're interested.