How to identify a layout placeholder programatically

My addin uses templates contained in particular layouts to set up the content of slides. Layouts contain a number of placeholders. My problem is that I don't know how to identify each of these placeholders on a slide to place the required information in them.

So far I have tried the following approaches:

- Naming the placeholders, and using Placeholders.FindByName: Unfortunately placeholders on slides are renamed, they do not maintain the name used in the layout.
- Using tags: Tags placed on placeholders on the layout are not copied over to the placeholders on the slide, so they can't be used to locate them on slides.
- Placing an identification text on the placeholders and looking for it on the slides: The problem is that text entered in the layout is shown in slides (before modifying it), but is not accesible through the TextFrame.TextRange.Text property.

What else can I try?