Since 7.3.2 MWG supports the automatic encryption of the data uploaded to Box, DropBox, Google Drive & MS One Drive. The file is encrypted during upload (after all rules are executed), and encrypted when user downloads file via MWG.
To use this functionality you need to import the "Cloud Storage Encryption" ruleset from Rules Library & adjust it correspondingly - select which password will be used for encryption, for which services it will be enabled, etc. (see product's documentation for more details)
Found it, thanks for giving me the name!