Ability to use criteria from child object when updating connected child records via record rules

Not sure if this is possible. When using record rules on a form submission, it is possible currently to update a field of all connected child records. It doesn't seem possible to update only one child record, probably because Knack wouldn't know which one to update. So would it be possible:

a) To have an option: Update latest child record?


b) To have a filter criteria on the child side as well. So currently the rule triggers when a field on the parent object matches a certain criteria. Could one add a criteria on the child side as well, e.g Update only if Field 1 is blank?

I've requested similar elsewhere that the record rules on form submission support more flexible 'where' criteria and more flexible set expressions.

eg. update connected 'invoice' records WHERE sales.order_date + 30 < current_dateĀ  SET invoice.status = overdue AND outstanding_balance = outstanding_balance * 102%

Okay a little contrived since this would be a nightly task maybe.