The Nintendo 64 shipped with a Memory Expansion Bay which contained a Jumper Pack. Some games such as Donkey Kong 64 and The Legend of Zelda: Majora's Mask required additional RAM to run, and shipped with a 4MB Memory Expansion Pak which was installed via this Memory Expansion Bay. Were there any other uses (licensed or unlicensed) for the Memory Expansion Bay? I only see Memory Expansion Pak accessories online (both from Nintendo and third-party) for this slot.
No, this is only for RAM. The jumper pack actually just terminates connection of the onboard RAM and adding the expansion pack works just as it sounds.
You can even upgrade the capacity to even more RAM than is supplied by the expansion pack and the console will boot with it, but no games will take advantage of it.
The RAMBUS bus was a very specific sort of thing, optimised for it's purpose of very-high-speed RAM, which was a black art then. So it wouldn't be suitable, or the best choice, for other hardware.
Besides which, any other hardware would be likely to mess up the functioning of the machine's internal RAM. The fact that a "jumper" (actually termination) pack goes there leads me to believe that there's only one RAMBUS channel on the machine. So anything plugging into that slot would've been connected to the onboard RAM's dedicated bus, which, as I mention, was very specifically tuned.
Modern PCs are the same. Early PCs had RAM and I/O devices all sharing the same CPU bus, but modern CPUs have specialised RAM controllers onboard, with dedicated pins for connecting only to RAM.
So no. If they had, it would've made the machine unreliable and cost a ton of money to adapt hardware to a specialised RAMBUS RAM bus, for no imaginable gain. Instead, you'd just put extra hardware in the slot underneath, the cartridge slot, or a controller port for slower speed data access. The cartridge slot would be a much friendlier place for electrical engineers! The underneath expansion slot was just an extension, same circuit connections, of the cart slot.