Of course this is possible, but it might get very tricky depending on the structure of the document you want to process and on the type of sample text you want to replace the content with. If the sample text should more or less look like readable text, you have to figure out a way to cut that text up into substrings and place them in the right locations so that the end result looks like the sample you are trying to create. It is easier if the output can be complete bogus, as you will not need to care about the length of the text strings that are replaced in each of the paragraphs or subparagraphs your script will find.
Lots of issues to handle. Possibly, using the Find and Replace function would be something to look at in the FrameMaker Scripting Guide. But that function is not exactly the easiest to handle from a script. The other option is to walk through all paragraphs in the flow, then walk through all anchored frames, figure out if they have paragraphs in them, then walk through all table cells and process the text in those.
An important issue is what to do with the many markers and anchors that FrameMaker puts in the text flow. These may or may not take up space in the text (depending on the type of marker), and you do not want to remove all of them. You may want to remove the index and cross-reference markers, but certainly not the anchors for tables and such. Also, walking through all paragraphs in a flow does not lead you through the text that appears in tables or in anchored frames.
Good luck
Jang