FFMPEG: Overlaying scaled video over image

I am trying to produce video containing static image and smaller video which would be overlaid on top of it. Dimensions and position of overlay video are dynamically sent to the server script which runs FFMPEG, so I need to scale video before overlaying it. So, numeric values I used are just for test. Original video has dimensions 72:48px. I tried command like this:

ffmpeg -i background.jpg -vf "movie=overlay.mp4, scale=128:96 [ier]; [in][ier] overlay=70:70 [out]" -y output.mp4

However, it just instantly produces an empty video. If, on the other hand, I try inserting -loop 1 right after ffmpeg command, the processing of video seems to take endless time and it never finishes.

What I am trying to achieve is to avoid creating scaled video first. Is it possible and what am I doing wrong?