As of #8189 there's preliminary support in the apple2ee driver for Rich Dreher's Compact Flash for Apple II card in slot 7. The CHDs it wants are 32MB ProDOS partition images with no partition header. Slot 7 drive 1 is the first 32 MB of the CHD, slot 7 drive 2 is the second 32 MB if any. You can find the necessary ROM for the card (which is open source, as is the hardware design) and sample HDD images on Rich's site. (Get the "Run 6 CDROM files" zip and you'll find the ROMs, source code for the ROMs, and 3 sample HDD images. Run the .po files through CHDMAN as uncompressed HDDs). Reading and writing appear to work fine, but be careful before running your business on emulated AppleWorks.
This would be more useful for the IIgs, but I don't have that driver behaving with the card yet. Nathan's original code wanted nothing to do with external cards in a IIgs and I'm still beating it into submission