I don't think this is much of a game. Perhaps it'd be better to handle it like a pinball machine - with auto-repeat for the keys. ie - hold down left on the remote to scroll through letters Z-A, and right on the remote to scroll through letters A-Z, and OK to choose the highlighted letter. I never use shifted/case on roku when entering anything, and symbols are a waste of time too unless you need to put in a WPA key. Numbers could be added to the beginning or end of the scrolling selection, and spacebar. Really that is the most efficient way I see of doing this - perhaps using the fastforward/rew to skip around by 10 symbols, ok to complete entry, and the back/instant replay/info button if available to delete.
Of course the ideal best solution would be to use a wireless IR or bluetooth keyboard. I can type on a keyboard, and enter text using the scrolling method mentioned above a lot quicker than 11.5 characters per minute.