Does Blender have a file locking feature? Is there some way to lock files with a password or to lock open files to prevent them from being edited by others?
Use case examples:
A:
Jack wants to sell Blender models, but wants to ensure that only those who have paid and been sent a password can edit the models.
Can a password be used?
B:
Jack and Jill are making a hill for their water simulation. However, Jack wants to make sure the hill is completed before Jill tries to run a water simulation.
(How) can he lock the file?
To be clear:

LibreOffice (also open source) allows password protected files.
I do not want to sell files as described, but I do want to protect my files via password sometimes (and will need to in some situations).
No, for the same reasons ideasman42 said in his own answer. He suggests some workarounds however, here are mine..

Case A:
You can use PasteAll.org (which has natural support for blend files) to safely upload and password protect a .blend file, you can then send the link and password to the customer.

Case B:
Easy enough, they could work in separate personal files and merge them when ready. Link/Append

Ultimately, you are better off using an actual software that provides encryption such as 7zip.