Nope, no custom keyboards.
After countless failed attempts to get the keyboard to show up I eventually decided to forcefully reboot the device (volume up + power button), after which the keyboard finally showed up again. So luckily I managed to unlock the device. Though I didn't know I could actually unlock the device through my.eset.com, that probably would have been a better option
Still, as you already mentioned, this is quite a nasty issue. Although I have only ran into this issue once so far (no problems on other mobile devices running EMS). Perhaps it's related to some other app running on the tablet (the tablet in question belongs to a relative, so I don't know exactly what apps are installed). I guess I could try to deliberately have EMS lock the device again to see if the issue reoccurs.