I remember reading somewhere that Frictional games released tutorials on how to make modifications and new maps for Overture and BP, but they also mentioned that no new maps were released due to the complex way of making the maps. Have no one ever given it a try?
No custom maps have ever been released. A user named WindexGlow used to be working on a mod called The Shaft, but he stopped. Not entirely sure why, but I'm almost certain that it had to do with the few development tools that existed being unintuitive and difficult.
I wish Frictional Games would look back at the Penumbra series, and see if they could make it easier to make custom stories.
The source code for HPL1 was released, but still it's outdated, didn't have much tools (you had to map the areas out in 3ds max) etc. Just use HPL2.