OK, I think the CMOS buffer option is likely best then.
A CD4049 would probably be a good choice, having considered it a bit more - they have input protection so just a 100K series resistor should be fine, and it will also give the inversion back to UART or MCU compatible polarity.
If you want to feed another RS232 level port, then a CD4050 should work the same for the resistor input side and drive most com port inputs as well, as they usually have a slight positive offset & will mostly take 0V as "negative" signal level.