I’m probably the biggest Dark Crystal fan around here, and I can say without hesitation George had no involvement with the script. Henson’s people did help out a bit with Yoda on Empire, as DC was years in pre production, and it was mutually beneficial to both parties. Lucas even tried to replicate Henson’s Creature Shop at ILM for ROTJ, with mixed results.

ILM did the matte paintings for DC, but that’s about it for any Lucas connection.

I welcome you to provide evidence to the contrary.