Having a detail record returned, even if it is trapped but empty, is the basis of Monarch output.
As far as I can imagine you have two options.
1. Have the detail template trap something in the records without detail even though the result will be blank fields. To be able to do this would be unusual in my experience.
2. Find a way to may the key fields from the append (or possibly footer but that is less likely) part of the detail record.
A third alternative would be to create a model that extracts and exports the required 'Append' records without concern for any other data.
A second process woudl extract the detail as you have it defined.
You then runs a third process, using the first extraction as the base file and the 'detailed' second extraction as a lookup table, to generate the file you really need. It will have all of the records and some extra detail for thos records for which it was available.
It's not as complex as it probably sounds if you use scripts and batch files.