I have modified a version of the thread tooltip preview plugin I found to work in myBB 1.6.3.
My php skills and knowledge of myBB's architecture is limited, but I was able to get it working and stable.
It has been running for a few months on a very busy forum (100k+ page requests per day) without any problems, so I am comfortable sharing the code now.


Note: Before proceeding with the installation, first backup your myBB database and uninstall any plugins which also provide thread tooltip support.


myBB Thread Tooltip Installation:

Step 1. Download the plugin here.

Step 2. Place the plugin in your ./inc/plugins directory.
 
Step 3. Login to your Admin CP and install the plugin. (Admin CP --> Home --> Plugins --> [Locate plugin] --> Click Install)

Step 4. Recount and Rebuild the thread tooltips cache.

Installation is complete.


Special Notes:

1. When when logged in as a Administrator or Moderator, you need to hover over the thread body to view the thread tooltip preview. This is because myBB allows admins and moderators to perform one click editing of the thread title.

2. When recounting and rebuilding the thread tooltip cache on large forums, it can take quite long to complete and puts the DB server under high load. If you receive an "Internal Server Error", start the rebuilding and recounting process again.


Credit: This version of the tooltip was originally made available by "ZiNgA BuRgA" for use on myBB 1.4.