USB mmcblk support (realtek or others)

Can a USB card-reader ever appear as /dev/mmcblk0 and support the MMC ioctl commands? I see there are some kernel driver modules such as: rtsx_usb_sdmmc.ko (rtsx-usb-sdmmc) rtsx_usb.ko (rtsx-usb) Can these make a realtek card reader, attached to the USB bus appear as a real MMC device? I notice that all the USB card readers I have access to appear as /dev/sdX. which appear to be an emulated SCSI device. These emulated SCSI devices do not support all the MMC operations (ioctl). Are there certain USB-attached card-readers (chips) that appear as a /dev/mmcblkX instead of /dev/sdX? This would allow the card to support blkdiscard, trim or fstrim.