I have not tried it myself, and I do not believe it will work due to (at a minimum) navigation issues, but you could give it a change to install the following plug-in on your BM5:
http://www.milliesoft.co.uk/index.php?option=com_content&view=article&id=72:spotify-mce&catid=1:software&Itemid=22
It is more or less the same approach you are using for adding additional codex's.
However, for spotify you need to logon. If the plug-in can remember your credentials this might work. Question than is how you can navigate. As stated, you can create "favourites" lists in spotify (via your PC), which you maybe can add to the corresponding favourites list of the BM5....
If you really would like to have this functionality, I think you could better try to run a tool like XBMC with the spotify plug-in on a Windows or Mac mediacenter.
Update: at the end of the BM5 wish list there is some discussion going on spotify as well (http://forum.beoworld.org/forums/p/33436/301969.aspx#301969)