The properties of a USB serial converter have nothing to do with the hardware of the hosting system, but only with the USB serial chip itself, and the software stack of the hosting system.
The pi should be using stock Linux USB serial drivers.
Therefore, you can enable/disable the linkage of DTR to port open/close via the usual Linux method of clearing the
hupcl setting as documented on the Arduino site and elsewhere:
stty -F /dev/ttyUSB0 -hupcl
Or replacing /dev/ttyUSB0 with whatever device file actually corresponds to your USB serial port (for example, the first Uno connected would likely be /dev/ttyACM0)
Even with regard to the PI's own native serial port, this behavior of DTR is ultimately under software control - anyone who argues otherwise is ignoring the fact that it is only the Linux driver, and not the hardware, which has any knowledge of the port being opened or closed. The actual port hardware can only tell that it is being read from or written to or reconfigured, none of which are actually synonymous with opening the serial device.