There’s no way I know of to do that. It would be like trying to figure out which InDesign files a particular Photoshop file is linked to.
If you can automate the process of exporting IDML from InDesign, and can automate searching within IDML (or renaming IDML to ZIP and extracting the contents and searching within those) in your OS, then you'll find that each linked INDD has a LinkResourceURI in the XML. I find 'em in spread_???.xml. I then use a regular expression (within Notepad++, assuming I'm working in Windows), to find each linked file and build up a list.
If you need to do this kind of update more than once, you may want to revamp your workflow to make stuff easier to track. Placed INDDs are great for single-use circumstances, but as you have found, it can easily explode into a version control nightmare if you don't have some resource-usage-tracking method in place.
I think what the OP is asking is for a way to take any random .indd file and generate a list of other files into which it was placed (preumably so he can update those?), sort of the inverse of your method.
I guess so... but if you were only running this on one INDD then you wouldn't need to automate the process of exporting 300-400 IDML files. Speaking of which, seems like I failed to add a handy link to my post, leading to a script that automates the process of exporting hundreds of IDMLs.
if you step through this for all 300-400 documents, you do get a list of every placed INDD and know where every placed INDD is used, which is a far sight better than no list of any INDDs and not knowing where any INDDs are used. What's worse, too much information or not enough?