Hi, I know that currently, Geany cannot edit multiple lines at once form the end of the lines when the lines have different length. I just found out a kind of glitch that allows you to do it just for one char. I just share it here.
1. Make a rectangular selection over the lines you want to edit and be sure to be on the right of the longest line. 2. Type the space key so that all lines will be padded on their right with spaces until the end of your rectangular selection. 3. While keeping the multiline cursor active (do not click anywhere in the text), then go to the menu and click on Document > Strip trailling spaces (not sure about the exact name in English). 4. You will see that each cursor will go at the end of their lines. 5. You are free now to type the char you want. 6. Unfortunately, once you have pressed a key, the cursor will go again at the end of the rectangular selection. 7. It's a bit tedious but at least you can add whatever you want at the end of multiple lines with different length in our favorite editor.
I am a big fan of Geany as a quick and simple text / editor. I am looking forward to see it implemented.