Is it possible to use the texture memory/cache (including the Expansion Pak) as additional (albeit limited) system RAM? If so, is that the case here?
There were some games that manually implemented their own virtual memory paging system to map the cartridge ROM to RAM address space.
But, in general a feature that distinguished the N64 from the PS1 was that it had a single, unified block of RAM for all of the hardware to share however you like.