There are functions for adding and removing files from the media database exposed in the SDK so it should be possible to make an indexer based on something other then an import directory, however, the first version of the SDK won't be very good for multithread operations like indexing.
The way I would suggest to do it now is since StreetDeck just uses the Windows Media Player database, is if you just write an external application that runs on its own when you start up StreetDeck to access media player and add files on its own, they *should* show up in StreetDeck since they both use the same database. In other words, if you just write an indexer for Windows Media Player 10, it will be an indexer for StreetDeck.
Files do NOT have to be in the _indexed folder to work in StreetDeck and they will only be moved there if they are indexed by the automatic indexer which only runs on files in the unindexed media path.
I haven't tested using media player concurrently exactly like this, but I have done other things like it and have a strong feeling it should work just fine.
I just made another thread explaining how you can automatically run files, such as your indexer, automatically everytime StreetDeck starts: