While I was already satisfied by the old version of the script I had, I stumbled upon the updated version (of aug 2012) on registry: http://registry.gimp.org/node/7385
Then I began to read the comments and someone pointed out, in 2010, how PS handled this high pass filter:
duplicate, blur, set layer blend mode to grain extract, merge down.
This looks more (to me at least) like photoshop's high-pass. It's more defined then the other one.
Then robA considered the comment and changed his script accordingly.
Reading this comment made me want to reproduce the filter manually, but instead of 'blur' I tried 'selective gaussian blur' (blur radius 5 & max delta 50) then overlaid twice, and this manual output appeared very good to me, especially because with the filter I had side effects making some dark areas darker (visible when dark contours are close to a dark area) and creating slight halos, side effects that didn't came up with the manual way. Looking twice, It seems the output I had was, above all, softer than the filter thus it would allow smooth high pass if needed and stronger if wanted by duplicating high pass overlay layer or using another blend mode.
Picture demo with a Public Domain photo: