Conceptually - yes, easily.

But to do it literally - I'm not a low-level motherboard EE, but I'd bet you're looking at 5 to 7 figures (US $) of engineering work, to get around all the ways in which that would violate assumptions baked into the designs of the CPU, support chips, firmwares, etc.

Make a fake ram which offers write through guarantee and returns bus no matter what address is referenced. You could possibly short circuit any "is ram there" test if it just says yes for whatever size and stride got configured.

The CPU literally initialises itself without DDR then initialises the DDR PHY, there must be a way of keeping the CPU in that "cache as RAM" mode.

Theoretically anything is possible with enough thought and work.

or an older version of puppy linux?