I don't think the option exists for GREP, but you don't really need it. You can just add optional diacritics to your GREP pattern.
Yep, ignoreDiacritics in findChangeTextOptions but not findChangeGrepOptions
You can use [[=a=]] to match 'a' and all accented variants (acute, grave, etc.). Same for all other letters. Could make for some awkward regular expressions, but you should be able to get it to work.
Assuming you want Hebrew diacritics, it would be something like this: