Hi
I have similar problem and now I am in fight with screen capture from my Tektronix AM700 Audio measurement set. Almost same situation, Tek has serial interface for printer (com1) with max speed of 38400 8,n,1 and when I push print I am able to get data to serial interface. After this, I try software "PrintCapture" on Windows system, and I get nice image.
xon/xoff must be switched on for serial, without I got garbage data and damaged image.
Now, I wish all this on Linux with Raspberry Pi. I think we can solve this problem with virtual printer from CUPS.
In nex few days I have to find some solution for this.
Cheers,
Damir