Aha, looks good, thanks. My own research shows that the actual Boxxle and Black Onyx carts appear to use 0xFF80 / 0xFF90 / 0xFFA0 / 0xFFB0 hotspots for EEPROM access though. Will check for ones at 0x6xxx as well.
Interestingly, that above appears to perhaps leverage what was mentioned in this post: "...a 74LS133 would be much more useful, since it has 13 inputs, which is great for decoding FFFx addresses..."
Respecting the final product, Pixelboy makes mention: "..we now have a ColecoVision PCB that supports 64K bankswitching (3 banks of 16K in the upper half of the cart ROM address space) and EEPROM support for data-saving..."
One other area that may be worth looking at is the $7800-$7FFF range. That was noted as being utilized for possible additional hotspots; however, nothing definitive stated as to whether or not that came to fruition.
Further along those llnes, the suggestion to make use of the entire $6000-$7FFF range was brought up in the discussion.