How I use it is to crop the 4:3 source so I just end up with the letterbox image, 2x resize with the edgeresample filter, rescale it again but with Lanczos3 to 1280x545 to get 1:1 aspect ratio image, apply a sharpen filter and then finally Lanczos3 resize & add black borders to get an anamorphic PAL image.

When your source is interlaced do you select "Interlaced (not recommended for enlarging)" in the resize settings?

Why is 1280x545 a 1:1 aspect ratio? Why do you want a 1:1 aspect ratio? It seems logical to me sharpening after the 2x resize with edgeresampler keeps quality better then resizing an extra time. So can you explain that?