I found that bluetooth keyboards worked fine and were my saving grace when I was installing ubuntu on my surface 2, so it may be your specific keyboard that is causing issues. Maybe try a different bluetooth keyboard?
I was using a generically branded Adapt ADK-100 keyboard.
Alternatively, you can try turning on the on screen keyboard. To do this, you'll need to boot into live mode, start the OSK from the accessibility settings, and then launch the install.
If you're using the microsoft surface keyboard, then it's not bluetooth, it uses their special side connector, and needs a kernel patch to get it to work (or did, the last few updates I've done haven't required a manual kernel rebuild, so we may now be ok).