Is it possible to create an empty bitmap with the TextureManager. ...
No, unfortunately not.
Why would one want to do such a thing?!
Considering roTextureManager is just image download manager.
Because it's not just a download manager. It also manages its own LRU cache, loading and unloading bitmaps from the cache as memory is needed. The problem is, it doesn't have any knowledge of bitmaps created outside of its cache. It just assumes that X amount of memory is available to it, and manages the memory based on that. If you have bitmaps created outside of it, it can overrun the available memory, causing issues. If you could register a bitmap with it, so it's aware of that memory being used, it could adjust accordingly. Of course, ultimately, it'd make a lot more sense for it to actually query available memory, instead of just assuming what's available.
Then again i am not sure why would one want to use roTextureManager either, seems all it does is use async downloads to tmp. Why did they give it "component" status and not just a routine in bslDefenderOfTehFaith or whatever that .brs "library" is? It feels like i am missing something here, what is it.
It doesn't just download the images asynchronously, it also creates (and scales, if needed) the bitmaps asynchronously. If, for example, you have a screen full of posters that you want to scroll through, creating all of those synchronously can have a significant impact on framerate.