Yeah, I like this idea. You can refer to items in the WHERE clause, so it seems like it would make sense to have an option which could refer to an item for the number of rows to write.
Slightly similar, one feature we get asked for a lot is to have the block group only insert rows that have data which has changed. As long as it has the row id (or one of the columns is an identifier) it should be no problem to make sense of the data.
This should make it in to the next major release (towards the end of the year…)
Regards,