Hi Linus,
I created a script a while ago that uses either the active book or the active document as starting point, and includes optional conversion of any text inset files to the requested FM or MIF version or to text and optionally copying the images to a new subfolder. I used it to create final source copies of my manuals for my customers in the FM version that they are using. The inclusion of text insets and images (from a large pool of images I store locally on my drive) means the customer can re-create the entire book from what I hand them, without any missing components. Just zip the new directory and send it off. I have tested all the capabilties of FM10 and FM11 and all the listed versions should work.
I am still working on an extension of the script that would check for incompatibilities when saving to an older FM format, Adobe just mentions that some features may get lost but does not actually check whether any of these features are included in your materials. I have not finished that part of the script yet, as that is quite a complex job to do, but without it you can get the script - without the source code - for free, if you promise to send me feedback. If you need the source code, we'll have to discuss a fair price for it.
So now you already have two options.
Jang