On it – there is an excellent USB3.0 → SDIO interface microcontroller available from NHK electronics ( Shenzhen ) that will do the trick. It’s a really tiny part ( mBGA!!! ) and low power The tricky thing will be to get the mechanical design tight enough to fit into the shell size for the expansion bay.
This will ONLY work for SD, high capacity SD, but won’t work with specialized SDIO cards like wireless expanders, or ones that export UART … but if you’re a photographer or someone that needs to dump full sized SD cards it should work.
If interested – please <3 this comment. That will help me determine potential demand for the expansion card and if it’s worth my time to develop and debug it.