How do integrated charger ICs dissipate differences in VCC and the battery voltage?
BQ2002 and similar ICs don't actually see the charging current path through them. They are just controllers, they don't regulate by themselves. They have an output (the CC pin) which is used to indirectly control the external, high-current passing element.

You can have a look to a reference design provided by TI. The CC output controls a LM317 which is used as the regulating element (where the thermal considerations indeed apply).

This way, you can have a much greater flexibility in your design (use whatever regulator you want, linear or switched, with whatever specs you need for your specific case).