Here's a TextExpander snippet (well, two snippets) for titlecasing the clipboard contents:
(Two snippets are necessary because of a bug that appends a newline to the end of any JXA snippets. This will be fixed in TextExpander 5.0.1, but in 5.0 it's necessary.)
In AppleScript, it is possible to get the system clipboard with something like the following:
set s to the clipboard as text
If you run this in Script Editor with “clip” on the clipboard, you should see this:
In JavaScript for Automation (JXA), you can achieve the same thing with the following code:
app = Application('System Events');
app.includeStandardAdditions = true;
s = app.theClipboard();
You can also use:
app = Application.currentApplication();
app.includeStandardAdditions = true;
s = app.theClipboard();