You can’t overwrite an original tif or jpg file from PhotoLab 5.
I have been doing some editing using a pixel editor (PhotoLine) and it allows me to call external editors as well as plugins and sends them the image to be processed. When those images are saved back to the original file name it (PhotoLine) reloads them so calling external editors is much like calling a plugin, but gives me a lot more flexibility.
I have mostly used the Nik apps and some Topaz apps but decided to try PL5. When PL5 loads the image that is sent to it it does not present a usable UI. That is, everything important is greyed out and so is not usable. But if I navigate away from the temp T folder where the image is stored and then navigate back using an alias I can process the image properly in PL5, but when I try to export it back to the same image (so it will be reloaded) PL5 refuses to do it. See the screen shot below.
I tried this using PL5 normally with both jpg and tiff images and found the same restriction. You can not overwrite original images.