Coreboot and some other low-level stuff uses cache-as-RAM during early steps of the boot process.

There was briefly a product called vCage loading a whole secure hypervisor into cache-as-RAM, with a goal of being secure against DRAM-remanence ("cold-boot") attacks where the DIMMs are fast-chilled to slow charge leakage and removed from the target system to dump their contents. Since the whole secure perimeter was on-die in the CPU, it could use memory encryption to treat the DRAM as untrusted.

So, yeah, you can do it. It's funky.

Where’s the market demand for that?

Where is the market demand for faster RAM? This isn't a good question.

Gamers, 3D modelling professionals, IBM, HP, ORACLE, many IT departments, etc… ?

Yeah, you’re basically betting that people will put a lot of effort in trying to out/optimize the hardware and perhaps to some degree the OS. Not a good bet.

When SMP first came out we had one large customer that wanted to manually handle scheduling themselves. That didn’t last long.

Effort? It's not like it's hard to map an SRAM chip to whatever address you want and expose it raw or as a block device. That's a 100 LOC kernel module.