These are certainly plausible, but awkward solutions. They require users to save everything on the Calories and Nutrition tabs to another file or open a separate browser window in order to hop back and forth between windows to manually enter that information into the custom food screen—the data is not in a format for bulk cutting and pasting.
Would it be possible to add a “make this day into a custom food” button that would pre-populate the calorie and nutrition totals you have entered for a day into the Nutrition Facts fields for a new custom food? Then all you would have to do is name the custom food. This is essentially the same as a recipe builder, and would make your example of going ahead a day to create a blank food log for “breakfast” a much more convenient option.