As an example, the N64 has a notorious 4kb texture cache. Could someone sap another 4kb (or more) from the system's RAM or the Expansion Pak and use that as texture cache space? If so, would it be worth it to do it?
Since the texture cache was dedicated hardware, read directly by similarly dedicated rendering hardware, you couldn't just point the renderer hardware elsewhere or relocate the cache.
You could stream larger texture data from the cartridge memory into the cache in 4k chunks (2k if using mipmapping), however, and that's generally what later-generation games tended to do.