I want to relate to you my experience with USB host mode, ask for recommendations on the most convenient USB keyboards you've found, and suggest some good hardware I've found.

What I did...
I spent several hours trying to get this working using the otg_mode modification, to no avail. The whole time I assumed the mod was probably successful, but that something was wrong with my mess of gender changers or power injection. Well, it turned out that (and I have since read this elsewhere) the otg_mode hack just wasn't sufficient. I HAD to use the flasher method instead. If you don't have a Linux machine from which to run flasher, this link explains how to do it from a virtual machine running under Windows. (NOTE: When I did this, I had to manually expose the 770 to the virtual machine by clicking on it in VMWare's VM->RemovableDevices->USBDevices menu.)

Once I tried this out and ran 'flasher --enable-usb-host-mode', everything just worked! I even had no problem mounting my 4gb thumb drive:
Incidentally, my current setup consists of OS2007HE running directly off of a 2gb mobile RSMMC card.


Hardware Configuration
1 @ mini-B (male) to A (female) adapter (attached to 770)
1 @ mini-B (male) to A (male) cable (between USB hub and bM-AM adapter)
1 @ noname miniature USB2 powered hub
1 @ USB2 mini keyboard (JME-8571 -- DOES NOT WORK!)

770 doesn't fully recognize this, so although most keys work, the caps/numlk/scrllk keys are stuck ON and consequently all of the letters that would normally double as a numeric keypad are stuck in numeric keypad mode. I have verified that it functions fine on a Windows PC though...

1 @ roll-up flexible keyboard

This works, but is very cumbersome, which defeats the purpose.

1 @ USB2 SD card reader (thumb drive that holds an SD card) + 4g card

I have to say that the cheap-o hub is PERFECT! It supplies power to the input port, so you don't have to inject power. Also, it's tiny. 1.75"x2.25". I highly recommend it.


Keyboards
The two keyboards I've tried are not going to work for me. The flexible/roll-up one is just impossible to use accurately. The other is nice and compact (8 1/2" x 4" x 1/2") and would be almost perfect, but I can't get it to be fully recognized (initialized?) by the 770, so half of the keyboard is stuck in numeric keypad mode.
So I ask your advice. If you have used a keyboard that was perfect, please let us know. Also, if you have a better understanding of the problem I'm having with mine, please share.