…via System Preferences
I can’t get it happening. Can you? 
Yes; are you sure you’re entering the menu item name exactly? (i.e., use an ellipsis instead of three dots.)
–Eric
No no! Not “Open Scene…”! (I can alter that just fine.) The “Open” in “Assets”.
…and System Preferences doesn’t seem to care about the option-semicolon vs. dot-dot-dot.
Works for me. Do you have something else in the menu which is also called Open, such as a script or custom menu item?
Wow, I went back to triple-check, and there it was, my shiny new keyboard shortcut, for xmas!!! Thanks, Santa Carter!
…I had restarted the OS/Unity since I made the original post. Who knows why it got garbled (I changed “Open Scene…” just fine, in the same short remapping session), but at least everything is good now. Weird.