Get a USB OTG cable, and connect a standard USB mouse to your phone. You can use the mouse on the lockscreen; no need to enable anything in advance.

I tried this with http://www.amazon.co.uk/gp/aw/d/B009DFCCKO and it worked perfectly with my Nexus 5. It took a few weeks to arrive from China, but luckily I wasn't in a hurry. I'm sure it's possible to get something similar sooner if you pay a little more.