The most straightforward way to do it is:
have strokeit post a message to an external program which is already running in the background
have the external program read the clipboard, change case, and put the result back in the clipboard
strokeit meanwhile waits 1/4 sec or so for the external program to do its stuff
strokeit then sends the ctrl-V keystroke
This requires writing the external program, which isn't really hard but isn't really easy either.
A kludgier way that would work if you have MS Word would be:
strokeit switches to Word (already running)
strokeit pastes the clipboard
strokeit sends keystrokes to word that change case and copy back to clipboard
strokeit switches back to original program
strokeit pastes clipboard