Nick answered my questions about this.
In short, OddsOn progressives don’t work like Microgaming progressives.
With an OddsOn jackpot, the numeric value of the jackpot is not the same across each currency. If an OddsOn progressive has a jackpot size of $100,000, someone playing in GBP would win around £50,000. And they would expect this, too. Someone playing in GBP would see a ticker that displays £50,000, and someone playing in USD would see a ticker that shows $100,000.