It is actually possible to have custom properties in the balloons specific to components in the assembly instead of editing it manually.
Basically, there are two ways to add a custom property to the balloon.
Link from the balloon Property manager:
When a balloon is created or selected, Custom Properties can be specified as Balloon Text values. This is from the Balloon Properties panel.
Link from Document property:
In Document Properties under the Balloons tab, the default text for a balloon can be set to a Custom Property as well. The value for this Custom Property setting, comes from a Property List.
Further this setting can also save as drawing template so that you can directly use the same settings to add balloons in assembly.
Custom property must be added on part level so that it can be reflect in the assembly. As in above example we have specified material property at part level.