Just hiding blocks from the editor and only make it "visible" within the document overview is a terrible idea and non-intuitive at all.

• it is not obvious that this happens, the content is just gone
• the document overview itself is not obvious, most of my customers don't know it (even after showing it) or don't use it.

Proposal

• Just leave the block within the editor and mark it somehow, maybe a "grayish hidden overlay" when not active

Here is a workaround to make it at least visible, when selected within the outline view:

.block-editor-block-list__block.is-block-hidden {
    visibility: visible;
    height: auto;

    &:not(.is-selected, :has(.is-selected)) {
      opacity: 0.7;
      filter: grayscale(1);
    }
  }