If you download the episode from the iplayer software on your pc, you could play it thru vlc (i'm sure you can) you may need to convert it to another format first though, the files should be windows media video
I will look into it later on when I've got a bit more time spare, if it will open in vlc then it will stream to the dreambox.
You won't get the proper downloads to work, since they're DRM encrypted. However, you can 'pretend' to be an iPhone, which allows you to get programs without DRM (the quality is as good as the flash streamed content, so it's not great, but it's the best you'll get). Have a look here for details. Once you've got the .mov file, use VLC in the usual way.
You might also be able to strip the DRM from the windows p2p files (for better quality), but I don't know if the latest versions are cracked yet.
Hope this helps.
edit: And as for a direct plugin on a dreambox, it's not going to happen. The DRM issue aside, Dreamboxes just don't have the CPU power to decode the codecs that the BBC uses. The only chance would be with the DM800 or 8000, but I doubt you'll see anything soon. There was talk of providing iPlayer services on set top boxes, but that seems to have fizzled out. You'd need mpeg4 hardware acceleration, which the current crop of dreamboxes don't have (AFAIK).