Just to give an update, I received my Female-Female USB adapter recently, and I used it to plug a USB keyboard into my n810. I used USB Control, and it gave me that message about the device being unsupported. Still, I had read that this message should be ignored, and that the keyboard would work anyway, so I went ahead and started typing away in Notes.

My findings just blew me away. Typing with a USB keyboard plugged directly into the USB port was just about as laggy as the bluetooth keyboard. When I was typing, I could see the CPU usage going waaaay up (from the process monitor applet). The words showed up as expected, but sometimes would display two or three at a time, just like with the slight lagging from the bluetooth. Has anyone else had this experience? Is there any way to fix this and get lag-free typing from an n810? I'd be very grateful for any hints.