[Arran]

One thing that many use with the diode dropping trick is to add a current limiting thermistor of some sort in series, like they used in series string TV sets. As the thermistor heats up the resistance drops cushioning the tube heaters since their resistance, or voltage drop, increases as the tubes heat up.
The dropping capacitor is the best way in my opinion, if you can do it, it acts as a soft start for the tube heaters. The problem being is that you need to find the right value of capacitor and you need also need room for it since a non polarized cap takes up a lot more room then an electrolytic of the same size and voltage. It's also hard sometimes to get a cap of exactly the right value, so you may still need to add a resistor in series to get the exact voltage.
Those metal can ballasts are really no more then a big wire would resistor, and they also burn out as you well know, I don't think there is much regulation with those types but still better then a diode. The ballasts with a glass envelope are better, they usually filled with some sort of gas to prevent the element from oxidizing and to disperse the heat, there may also be more voltage regulation due to this.
If you are interested in preserving an older AC/DC radio, removing as much heat as possible from inside the cabinet would be a good idea. Heat and electronics are a bad mix in general, and sets with ballasts get hotter then just about anything else.
Regards
Arran