You could do this using an Opus rename script but I think in this case, unless you're a VBScript/Javascript wizard, it'd be a lot easier to use a text editor. It's all being done on the clipboard in memory, with no files involved, so Opus isn't providing much other than an easy way to trigger a global hotkey.
You could instead have a global hotkey which opens a new text editor window and then use a few simple keystrokes to do the job.
e.g. With TextPad (since it's what I have installed):
Ctrl-V, Ctrl-A, Ctrl-Shift-K, Ctrl-C.
(Ctrl-Shift-K is the hotkey for Edit -> Change Case -> Sentence Case)
Any good text editor should let you do that, though the Sentence Case hotkey/menu item will vary.
Going back to the idea of doing it via VBScript/rename scripts:
It's a pain to get a large amount of clipboard text into VBScript. You either have to pass it as an argument, which has length limits, or you have to use the IE COM object to grab it, which creates an annoying security prompt you have to click.
Since there isn't a good standard "set clipboard" object either, to set the clipboard text you have to call out to either Opus's Clipboard SET command or something like my SetClip.exe.
It is possible to do in scripts, and some of the rename script examples do do it, but in this case where you're not passing any files to the script, I think it'd be easier just to use an editor...
If you do want to go the script route and need help writing it, though, let me know and I can point you to the most useful examples.