I think adding supplements at a much later stage (or at any stage) in your fast, to prevent the onset of an electrolyte deficiency, is really delving into the unknown. My personal opinion is against the idea as this will interfere with the fasting process and is effectively breaking it.
The improper breaking of a fast has been known to result in death or serious complications. HOW a lengthy fast is broken, is just as important and delicate as the fast itself.
The only safe way of adding supplements at the later stage is by qualified supervision and even then the advice would probably be not to.
You will definitely accomplish a great deal of healing and cleansing for a fast of 20 to 40 days so this should be more than sufficient (in my opinion).
If anyone knows a method of measuring the quality of someones food reserves, then I would like to hear of it.
Your body should warn of the danger of low electrolytes by the return of hunger, but that wasn't the case when Dr Joel Fuhrman MD conducted his own 46 day fast, and terminated because of low E's. This also happened to me on my first extended fast which again was terminated for the same reason (they said): but I felt fine. Maybe fasting supervisors err on the side of caution too much!!