I love to hear all this talk, I am looking for the same holly-grail.
Here are some additions to the discussion.
TiddleyMultiUser single executable (Jeds is brilliant, and with the improvements mentioned hear is very cool) this is even before I have exploited the MultiUser features)
As jed points out you can have it running in more than one tab or browser safely, but another advantage is you can spawn multiple sessions against the one wiki, to have multiple views of the same wiki
A lot more will follow
TiddlyServer latest version allows folder wiki creation in the browser
All it needs for single file wikis is to have the upload file make copies not overwrite, and point to a nominated list of editions
A possible much neglected solution of Windows (other platform?) is renaming a single file wiki to .hta hypertext application
I am also working on renaming a singlefile wiki to *.aspx to use on sharepoint (Tip save in WebPages library)
NoteSelf is also very powerful allowing hosted solutions with data saved in each users browser.
Wikis can subsequently be saved or have user updated plugins exported.
On thought is to allow user updates to be emailed as a json file from the noteself wiki as a submission process, to be imported into a consolidated wiki.
These are exciting times, and my not fully articulated dreams are coming true as new possibilities multiply, no increase exponentially.
Next? Members of the community collaborating to provide services even income on top of the open platform.
Regards