Thanks for the answer.
Well.... calling the script worked, but it does not solve my problem.
Here's what I found (I might be wrong, but I did a lot of testing):
1) UltraEdit only calls the script correctly if in a new UltraEdit instance. Checking allow multiple instances works;
2) The first argument must be a file to be opened. calling uedit32 /s=path does not work and the /s argument won't be interpreted correctly (it opens a file with the name "=path"). So it has to be
uedit32 file /s=script
3) Only one option can be passed in a single call. So even though /fni opens a file in a new instance, I can't specify /fni AND /s. Apparently that's why the only option is to check allow multiple instances.
So... It seems to me that this "must be in other instant" issue is a bug.
Besides, since I can't call the script in a different UE window and write something to the output of the old UE window (automatically I mean), this does not work for me.
Anyone has other ideas? Or maybe I should just send a suggestion to the development team...