I was wondering, since it is possible to install scan-doubler on a physical Amiga, is there a way of emulating this with UAE4ARM? From what I recall you select interlaced modes on Windows versions of UAE and still get a stable progressive output.
Perhaps you can go around this by connecting the Raspberry Pi through RCA on a modern TV (since they usually have built in scan-doubling on the analogue inputs), and selecting a standard PAL/NTSC output from the RPi. However, the degradation in image quality would be be pretty severe.