Unless I am missing something - currently, when you want to merge or append a value(s) to an array in field of a record in EDIT RECORD - you have to GET the record first.
Then use dot notation from the response of the GET RECORD call for the value of the array in the EDIT RECORD - and merge/append, etc. on that with a filter.
Seems like it would be more slick if you just had an option for 'existing array' or something to that effect in the EDIT record - so you could just apply merge/append, etc. with a filter(s) onto the existing array - vs having to do the step of GET RECORD.