I am not aware of a specific search engine that would allow this. The condition that secluded and quite places don't go together with those masses of tourists that would support the business models of the different aggregator services like booking.com, etc.
What works for me though is to use Google maps to find accommodation in rural places. The steps involved are as follows:
- Identify the region you would like to visit. (e.g. Monshau)
- Set the range you are willing to include in your search (To stick with Monshau enlarge the map to include Aachen and Stavelot).
- Identify the name of your preferred type of accomodation in the local language (again in the Monshau example being either Ferienhaus (German) or Chambre d'hotes/Gites(French)
- Type in either gite/ferienhause or both in the just enlarge area in google maps and the different accommodation options will pop up.
- Contact the different options.
The most difficult part might be contacting the person offering the accommodation. Not everybody is fluent in English, but then this feature might support the "inaccessibility" of a region that makes it bucolic (like Mark this is a new word for me).
In France you want to search for "Gites", "Chambre d'hotes", "Tourisme Rural". In German so far I have only identified "Ferienhaus" (I would appreciate other synonyms). In Italian you could search for "Agriturismo".