Thanks people - will try Prawnsushi’s example too, sort of like what I’m looking for…
Garagecoder’s ‘Exfusion3’ filter seems to be as close as I can get, using the commands…
gmic v -99 str_0000_000000.png str_0001_000001.png str_0002_000002.png str_0003_000003.png str_0004_000004.png exfusion3 0.3,0.3,0.2,0.3,-3,0 output zzx2_0000.tiff
gmic v -99 str_0001_000000.png str_0003_000001.png str_0005_000002.png str_0007_000003.png str_0009_000004.png exfusion3 0.3,0.3,0.2,0.3,-3,0 output zzx2_0001.tiff
gmic v -99 str_0002_000000.png str_0005_000001.png str_0008_000002.png str_0011_000003.png str_0014_000004.png exfusion3 0.3,0.3,0.2,0.3,-3,0 output zzx2_0002.tiff
etc