Hello Scott,

Windows has a built-in driver, Usbccid.sys, for CCID (Chip/smart card interface devices).
"USB FAQ:Introductory Level - USB Class drivers" on WHDC
"USB Drivers Included with Windows" on Lakeview Research
"Chip/Smart Card Interface Devices" specification on USB.org

Next link shows a CCID firmware for ST7SCR written in 'C'.
"USB CCID - ST7SCR Evaluation Board Firmware v1.3.0" on ST Micro

And this link explains the usage of MMC.
"AN189: MMC Data Logger Example" and AN189SW on SiLabs

Combining these information, any MCU which has USB and SPI, such as Cypress EZ-USB, SiLabs 'F32x, will do your job.

Tsuneo