Thanks Fernando!
I think I may have found a bug. I’m able to retrieve the value when using GP Premium’s Dynamic Options but not when I’m using GB Pro’s Dynamic Data. With the GB metabox, the value seems to be ignored, as if the custom field didn’t exist.
The only drawback is that with GP’s dynamic options, I don’t have the option “Remove block if link is empty”, and it’s pretty essential in this case. I can’t display a button that has no link :/