Could you allow the Roku device to learn to use unused buttons on your existing tv remote for navigation and control.
Many tv remotes have controls on them for another device that aren't being used. You could allow the Roku unit to assign a Roku remote function button to an ir code learned.
It might work by showing on the screen a virtual representation of a Roku remote, and when you select the virtual button (for example the UP button) you are instructed to press a button on the TV remote you want to have that function. The Roku then confirms it's learnt that button and pressing that button on your TV remote now makes Roku do the UP function. This would repeat for essential navigation home and back buttons at a minimum. The original Roku remote would always continue to work.
This would be awesome function to minimise lost remote problems because your tv remote could be augmented to control Roku and it would work for every existing remote and every Roku.