Can you or anyone else get a finger inside the HF? If not why does it have to be sanded. If you can then you have the have something like a pair of forceps that you can hold sandpaper with and work that inside.
Splinter brings up a good point. Sanding the inside is for other HF turners. Having said that, I've done it two ways. 1) with forceps to grip your sand paper and 2) with broken tempered glass or other abrasive media. Put the tempered glass inside, plug the end and let it run on the lathe over night at about 100 RPM. Acts as a rock tumbler type thing. You will need to cool your lathe motor as it isn't going fast enough to cool itself. Hope this helps.