I was assuming a single name option too. For indices I’d just use implied information from the brokers market - for spx you can find div swaps or forwards, for stoxx you can also use the div futures.Depends on the market. I would value spx divs flat to up as payouts are more stable. For sx5e I would project earnings and use stated payout ratios. This is as a buysider looking for an opportunity.
As a sellsider, I would mark against the longest dated that I can hedge to and then shock the forward div to give me a confidence interval to quote around.