I was in the roofing industry and went to technical meetings about roof coatings. Although there are now products marketed for coating shingles, there are reservations about this. One of the main ones is that if water gets through somewhere, it can get trapped underneath and not evaporate because the coating will make the shingles breath less. This could lead to premature rotting of your roof decking.
I would only try this if you positively cant afford re-roofing and the shingles are still in good enough shape to be coated good (no moss, etc. )