I don’t know if there’s a cross-platform API that would support this, so if there were a PD external that could output keystrokes, it would likely have specific implementation for each of mac/windows/linux.
If I remember correctly, in Typosaurus, we used the CGEvent api (CGEventCreateKeyboardEvent, I think?). I’m completely unfamiliar with Windows APIs, so I don’t know what the equivalent might be.
At the time, Typosaurus was rolled into a custom build of MonomeSerial, as a special mode, since MonomeSerial already had the scaffolding to interface directly with the FTDI serial driver. Doing this via SerialOSC would mean that you need to build a standalone app that speaks Bonjour/OSC in order to connect to SerialOSC, and then performs the mapping you suggested, to whatever system API is available for generating virtual keystrokes.
edit - it looks like keybd_event does what you want on Windows, but I have no idea if Win32 is still a good API to use