ImageJ can communicate via a serial port, and my interest is in sampling an analog voltage when I snap a camera frame and adding alphanumeric data to the image, but I haven't seen anything about communicating through a virtual serial port (USB) to the port on the Uno.
The relationship between the virtual serial port that the PC applications talk to and the Arduino's serial port is handled by the driver on the PC and the chip (FTDI or other) on the Arduino.
Have ImageJ talk to the serial port that the Arduino is connected to. Have the Arduino talk to the serial port. Instant communication.
but I've not been able to find a solution that doesn't employ creating a for-real RS232 interface for the Uno and tying it to a for-real serial port
How are you programming your Uno? That process doesn't involve a for-real RS232 interface and a for-real serial port.