these custom pages could have the custom field prompt, maybe hidden by default. Try to enable it clicking on the upper right “screen options” button, and selecting the “custom fields” checkbox. Cheers!
The author of the ivy cat testimonial plugin kindly gave me code to add the custom field functionality to his pages. I have your really_simple_share_disable=yes custom field in each of the testimonial pages, but the buttons still show up on the first testimonial. The testimonial plugin is ivy cat testimonial which rotates the testimonials in the widget area.