Is there a downside to having fl added to one's water supply...to the bones or elsewhere--accepting that topically fl applied to teeth is beneficial?

Only if in excess: The best method to have Fluoride incorporated into the tooth structure to help strengthen them against cavities is through systemic means as with public fluoridated water. The only downside would be if the total Fluoride ingested would exceed recommended levels. Therefore, either through water supply or Fluoride supplements but not both! it would cause enamel mottling.

Fluoride: Adjusting the Fluoride level in drinking water to the optimum level has virtually no downside. The maximum dental benefit is derived at the time teeth are being formed from birth through about 8 to 10 years old. It also helps strengthen bone throughout life. Topical Fluoride application to teeth has temporary benefit, and must be reapplied periodically because it eventually leaches out.