SD library API for use with SDIO port

[PaulStoffregen]

All SD cards support 2 access methods, native SDIO mode and a simpler/slower SPI mode meant for microcontrollers. Some newer 32 bit microcontrollers are featuring native SDIO ports. I'm working with one right now....

Would like any input on how the SD library API should be extended to support these? So far, my main thought is to define a symbol like SDIO_PORT

  SD.begin(4);          // use SPI mode with pin 4 as chip select

  SD.begin(SDIO_PORT);  // use native SDIO port, which does not have a chip select line

Does this look reasonable? Or has anyone already made other plans or ideas for supporting chips with native SDIO?