OK, v1.4 of the plugin is up and includes app and DLC-related commands from ISteamApps.
A DLC example project has been added and the wiki has been updated with the new commands.
Added Commands
IsVACBanned
GetAppBuildID, GetCurrentBetaName, GetAvailableGameLanguages, GetCurrentGameLanguage, IsAppInstalled, GetAppInstallDir, GetAppOwner, GetEarliestPurchaseUnixTime, GetInstalledDepotsJSON
GetLaunchQueryParam, HasNewLaunchQueryParameters
IsCybercafe, IsLowViolence, IsSubscribed, IsSubscribedApp, IsSubscribedFromFreeWeekend, MarkContentCorrupt
IsDLCInstalled, GetDLCDataJSON, GetDLCCount, GetDLCDataByIndexJSON, InstallDLC, UninstallDLC, GetDLCDownloadProgressJSON, HasNewDLCInstalled, GetNewDLCInstalled
@IBOL: The is no direct "GetSteamDLCOwned".
If you just want to see if DLC is installed, use IsDLCInstalled with the DLC App ID.
If you want to see if the current user owns it (installed or not), use GetDLCDataJSON, find the index with the matching DLC App ID and check the Available value. If 1, it's owned, 0 means not owned.
This can't be tested with Spaceware/App 480. It has two DLC packages (plus a hidden one), but they can't be installed (says I don't own them and can't figure out how to add them to my account).