Using "Add another" field widget on non-fields (extra_fields or Entity API properties) I would like to have a multiple-value property attached to an entity.

Currently, all of the properties are handled with field_extra_field and then stored in a single database table so we don't add the overhead of lots more tables. We don't need all of the neat features of Field API like versioning, so this works.

However, we do need one feature, the multiple value handling. Is there any way to get this without making a real field?
The Examples project has an "ajax example" submodule that shows how such a thing can be done with #ajax in a form. Should get you far.