I have never heard of a designated POTUS ecept when one was under the knife/near death. Ever. Last weekend, 0bama went AWOL. That was not OK.
In September 1919, Woodrow Wilson had a stroke that essentially left him in the produce aisle. His first lady, Edith Wilson essentially ran the executive department from the shadows, subverting the sitting VP and dealing directly with the Cabinet Secretaries. Certainly it was unconstitutional and without precedent, but alas, the Wilsons were progressives, so the rules needed not apply to them.