Is there a way to export / extract a document's used data merge placeholders?

Question

Is there any way, by feature, scripting or XML trickery, to extract a list of the Data Merge placeholders that are actually used in an Indesign document?
InDesign clearly has this data - it uses it in the Data Merge window to tell you which pages the listed placeholders appear on. Is there any way to get the list of used placeholders as a CSV, list, text file, etc etc?
I'm looking for a way on a very complex Data Merge project to ease fixing errors and to ease paring the data sheets down to only the fields that are in use (out of hundreds).

Answer 1

The problem really is that one person has InDesign and the other has Photoshop.
The workflow is images from photoshop are placed in Photoshop.
Not the layout from InDesign is placed in photoshop.
That's how the software is designed to work.
What people are saying is that Photoshop cannot work this way in dealing with layouts, it's a nightmare and will give you terrible results and cost you time and effort when updating things in Photoshop, it has all the makings of a Final Destination movie, particularly in the area of the later sequels.
But let's not get hung up on those franchises.
If the person needs to edit an Indesign file and can't afford the full cost of InDesign then there are other workarounds:
1. If it's just text
If all they are doing is updating text then you can use File>Export>Rich Text Format from a selection of text, this will make a Word version of the text that they can edit. You can then use File>Import and import the RTF that is edited.
2. You can use InCopy
This is a viable solution for editors that need to see the text in the Layout Format, but it hasn't got all the bells and whistles of InDesign, but it works in tangent with InDesign, it's not as pricey as InDesign and it also allows you to lock out editors from editing certain parts or moving things within the layout. And it can update your InDesign file on the fly.
3. You can use Rorohikos Story Tweaker
http://www.rorohiko.com/wordpress/2009/03/25/storytweakertextedits/
There are workarounds that you can find for people that don't have InDesign. It's just that the Photoshop workflow isn't a viable solution in any sense.
For example, I have my layouts proofed in PDF format, where the proofreader adds Comments to the PDF. It works for our workflow as I'm the one editing the only file in existence.
But to go from InDesign to Photoshop is a complete  waste of time and effort on everyone's part. There is no need to go that route, there's no reason, there's no point.