What platform, Vision or Perspective? What are you binding the template instances to, tags? If that is the case, the proper approach is to pass the tagpath to each instance using parameters and use the path to indirectly bind a custom property on the instance to the tag.
The following forum posts may provide insight.
For vision:
For perspective:
Searching for indirect tag binding on the forums will give you many more posts and examples. Consider also consulting the manual and inductive university (free).