You are right: The screenshot I presented definitely shows a bunch of "undocumented" commands. At least I couldn't find any symbols for some _tagpropertykey values the A2400IS is reporting. I'll attach a string version of it in case it might be valuable for someone.
Right now I have implemented some basic commands like enumerating devices, device content and various properties as well as uploading, downloading and deleting content. All this works well with other WPD devices. Just the A2400IS (which uses the MTP 1.0 protocol) is behaving somehow strange concerning uploads.
Digging further on I'll port more and more information retrieval and functionally into a usable COM wrapper. That might give me more insight.
@reyalp: I'll keep that in mind. Although I haven't stumbled over commands of this type yet, they might be dug within the automatic COM interop wrapper I'm using.
// capabilities reported by an A2400IS
WPD_COMMAND_OBJECT_ENUMERATION_START_FIND
WPD_COMMAND_OBJECT_ENUMERATION_FIND_NEXT
WPD_COMMAND_OBJECT_ENUMERATION_END_FIND
WPD_COMMAND_OBJECT_PROPERTIES_GET_SUPPORTED
WPD_COMMAND_OBJECT_PROPERTIES_GET
WPD_COMMAND_OBJECT_PROPERTIES_GET_ALL
WPD_COMMAND_OBJECT_PROPERTIES_GET_ATTRIBUTES
WPD_COMMAND_OBJECT_RESOURCES_GET_SUPPORTED
WPD_COMMAND_OBJECT_RESOURCES_GET_ATTRIBUTES
WPD_COMMAND_OBJECT_RESOURCES_OPEN
WPD_COMMAND_OBJECT_RESOURCES_READ
WPD_COMMAND_OBJECT_RESOURCES_CLOSE
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_COMMANDS
WPD_COMMAND_CAPABILITIES_GET_COMMAND_OPTIONS
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_FUNCTIONAL_CATEGORIES
WPD_COMMAND_CAPABILITIES_GET_FUNCTIONAL_OBJECTS
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_CONTENT_TYPES
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_FORMATS
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_FORMAT_PROPERTIES
WPD_COMMAND_CAPABILITIES_GET_FIXED_PROPERTY_ATTRIBUTES
WPD_COMMAND_CAPABILITIES_GET_SUPPORTED_EVENTS
WPD_COMMAND_CAPABILITIES_GET_EVENT_OPTIONS
pid=11 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=18 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=12 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=13 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=14 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=15 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=16 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=17 fmtid=4d545058-1a2e-4106-a357-771e0819fc56
pid=11 fmtid=b520e0c2-41df-4ebd-93e7-5b99c101932f
pid=12 fmtid=b520e0c2-41df-4ebd-93e7-5b99c101932f
WPD_COMMAND_OBJECT_MANAGEMENT_DELETE_OBJECTS
WPD_COMMAND_OBJECT_MANAGEMENT_CREATE_OBJECT_WITH_PROPERTIES_ONLY
WPD_COMMAND_OBJECT_MANAGEMENT_CREATE_OBJECT_WITH_PROPERTIES_AND_DATA
WPD_COMMAND_OBJECT_MANAGEMENT_WRITE_OBJECT_DATA
WPD_COMMAND_OBJECT_MANAGEMENT_COMMIT_OBJECT
WPD_COMMAND_OBJECT_MANAGEMENT_REVERT_OBJECT
WPD_COMMAND_OBJECT_PROPERTIES_SET
WPD_COMMAND_STORAGE_FORMAT
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_LIST_START
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_LIST_NEXT
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_LIST_END
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_FORMAT_START
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_FORMAT_NEXT
WPD_COMMAND_OBJECT_PROPERTIES_BULK_GET_VALUES_BY_OBJECT_FORMAT_END
