I have a temporary fix for anyone who is still having this issue. Haven't tried the other fix because I'm planning on getting an upgrade soon and it doesn't happen often but here's the workaround:
When keyboard is stuck on lock screen blocking your pattern:
1. Swipe from the top down to open your notifications panel (the slide down menu). Even if you have none, it will show options to shut off WiFi, etc.
2. Swipe from bottom to top to make the notifications go away. You will see the emergency call and quick-picture icons appear briefly. Make sure to tap quickly on the screen to make them re-appear if they disappear to make them pop back up. If you miss them and your screen goes black, repeat steps 1 & 2.
3. Slide either one (I usually default to the picture to avoid accidentally making an emergency call) and it should open up the respective app.
4. Press the back button to exit out. This will take you back to a black screen. When you re-open the lock screen, the keyboard should be gone
Let me know if this helps
EDIT: I had initially thought this was for GS4, but realized this was in a GS6 thread. My apologies if this isn't applicable to GS6, but it might still work. I can confirm this works on the GS4.