Is it possible to copy from SD card reader to USB memory stick directly?
Found a solution after further research.

Install app USB OTG Helper which requires BusyBox to be installed. I believe both apps require root to function properly. After that I attach an unpowered USB hub with multiple ports. Connect the card reader, and tap on "mount" in USB OTG Helper. Connect USB memory stick, again tap on "mount". Then I can copy directly from the card reader to the stick using any file manager.

Note that the mount points the Helper app shows are incorrect. It claims both drives have been mounted under /storage/UsbOtgDrives, but one of them was under usbdisk instead.