Hey Dave,
I'm afraid not. As you've mentioned, while Workplace Search can extract content from various presentation formats like PowerPoint, the indexed content doesn't retain any metadata like slide number that you could use at either search or retrieval time.
In general for non-plaintext document types like presentations, the extracted data is normalized into plaintext, and so the data you have available at search time generally doesn't contain those format-specific markers.
Hope that clears things up.
Ross