I apologize if this has been asked before, but does the N64 adapter support Rumble Paks?
When I added firmware support for the N64 Controller Paks (memory cards) years ago, I also tried to implement
detection of Rumble Paks. But I didn't get it to work. My Rumble Pak was always detected as an empty memory card.
I'm not aware of anyone trying to add Rumble Pak support, so it probably still doesn't exist in the Retrode firmware.
To implement it, one would need to know what commands the Rumble Paks accept (transmission of commands should work the same as for the memory cards), and also how to make the rumbling functions available through the USB HID interface so that they work with appropriate APIs (Force Feedback?). And finally, emulators would need to use those APIs to emulate Rumble Paks. Even if all that was done, the rumble might still not feel the same (literally) as on original hardware.
Another solution might be to work together with emulator devs and create some kind of tunnel through the Retrode for an emulator to communicate with an N64 Controller and its attachments directly. This would provide the most accurate experience, I guess.
Sadly, I don't have time to explore this interesting topic any further. Anyone else?