Call me radical, but have you tried a sachet of kettle descaler? I presume it'll work as my kettle's plastic and it doesn't damage it. You can buy it places like Woolworths, Robert Dyas, local pound shop etc.
You could try vinegar as a cheaper - smellier - alternative. If the cage is to be used - once again - by hamsters then it may be a safer alternative. Incidentally, are you sure the encrustation is not urea from the previous occupants? I cannot see why limescale would build up in a hamster cage
I've heard of the vinegar one as well. It probably is urea but I've unsuccessfully tried all sorts of Cream cleaner to get rid of it so assumed it was limescale
Have you tried making a paste from bicarb and smearing it onto the crusty lump? Kettle descaler might be a bit on the drastic side - any residue left even after rinsing might dissolve the hamster!!