It's almost eerie that I'm working on something similar on this very night!
But, from your description thus far, your task is simpler than mine.
If your webhook endpoint can itself keep track of the pointer (current array index) then store the value as a JSON array of values. JSON is a standard notation and more adaptable than, say, a comma-separated list. Send the JSON to the endpoint and have it iterate over the list before returning, and also have it store what index it last processed. Assuming the field is only appended to (not cleared) than there's no risk of reprocessing.
Thanks for the help !
I am stuck at a stage before the webhook call happens, that being, whether a custom lead field in marketo can accept arrays as data value.
Thank You, for the help.