Hi João,
Thanks for reaching out. It is not possible to retrieve the MostSpecificId field from the Campaign Location Target Report. However, the Id field in the Campaign Location Target Report is the Location Attribute so you can use it and do not need to look for MostSpecificId and get location data through LocationCriterionService. Please let me know if you have any further concerns.
Thanks and regards,
Xiaoming, Google Ads API Team