I've just checked : according to the IBM site, there are four models of 20Gb. (see here)
Of three of the models, the datasheet can be found. All drives have a physical height of 9.5 mm!

The last drive that doesn't have a datasheet is a 5400RPM model. I dont believe the Empeg guys ever used those in their players. Wouldn't make much sense too, very overkill....

So considering you can add drives up to 14mm, I'd say you're pretty safe!
Feeling better now?