April 28th Update - Back to drawing board. :o)
SPI/SDCard status- I am able to read blocks on both a 32MB and 128MB MMC card. I am also able to read block 0 on a 1GB MicroSD card (have not determined the issue reading other blocks yet). I have not been able to properly init communications to a 4GB MicroSD card. These are what I have on hand to test with at the moment. I have started on a simple RAM resident ProDOS block driver to make it easier to test both reading and writing.
SRAM Status - I have been considering some feedback given on comp.sys.apple2 to my project. One suggestion from Michael Mahon was to ditch the Slinky emulation and support Ramworks (Z-ram for example) style AUXmem augmentation. This would make it compatible with more software and free up the ROM space of the Slinky driver for use by a SDCard ProDOS block driver.
I started to investigate how AUX memory works and what might be involved in subverting it in the //c . Since my development platform is the //c+ I also took notice of the additional signals (EN80 and INH) available on the connector located beside the memory expansion port. From what I can tell so far, it may be possible to implement what I want without having to tap the MMU socket directly (at least on a //c+). ROMEN1 Is also available which means I could override the onboard ROM with out having to tap into it. I am still working this out (with the help of Jim Sathers Understanding the Apple //e at my side)
So the next steps as I see them are to 1) get the existing slinky hardware working rock solid on the //c+ so I have a good foundation in which to try and implement the AUX style memory interface. 2) add support for AUXmem override.
It may be possible to support both AUXmem and Slinky with the same hardware and a different CPLD code load for each type. (It would also be kind of neat if it could be partitioned for both simultaneously - with a big enough SRAM of course)