You can use the DIMM slot for the DDR bus interface, but not used in the traditional way DRAM would be. The DDR interface is extremely fast, and can be purposed for uses other than the standard row/column/select/precharge etc commands.

You would likely need chipset support for it to be used in this fashion, though.

That's what flash DIMMs do. The problem is that if you stick something in a DIMM slot people expect it to behave like RAM, not like an I/O device.

I don't think the kind of person who buys a DIMM SSD is the type of person who is confused by that sort of thing.

To a non-technical person, mSATA, DIMM, and PCIE SSDs all look the same. These products aren't for them.